Miniaturization of UWB antennas on organic material Nikolaou, S., & Abbasi, M. A. B. (2016). Miniaturization of UWB antennas on organic material. International Journal of Antennas and Propagation, 1-12. [5949254]. https://doi.org/10.1155/2016/5949254 Published in: International Journal of Antennas and Propagation Document Version: Publisher's PDF, also known as Version of record Queen's University Belfast - Research Portal: Link to publication record in Queen's University Belfast Research Portal Publisher rights Copyright 2017 the authors. This is an open access article published under a Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. General rights Copyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made to ensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in the Research Portal that you believe breaches copyright or violates any law, please contact [email protected]. Download date:07. Sep. 2021
Transcript
Miniaturization of UWB antennas on organic material
Nikolaou S amp Abbasi M A B (2016) Miniaturization of UWB antennas on organic material InternationalJournal of Antennas and Propagation 1-12 [5949254] httpsdoiorg10115520165949254
Published inInternational Journal of Antennas and Propagation
Document VersionPublishers PDF also known as Version of record
Queens University Belfast - Research PortalLink to publication record in Queens University Belfast Research Portal
Publisher rightsCopyright 2017 the authorsThis is an open access article published under a Creative Commons Attribution License (httpscreativecommonsorglicensesby40)which permits unrestricted use distribution and reproduction in any medium provided the author and source are cited
General rightsCopyright for the publications made accessible via the Queens University Belfast Research Portal is retained by the author(s) and or othercopyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associatedwith these rights
Take down policyThe Research Portal is Queens institutional repository that provides access to Queens research output Every effort has been made toensure that content in the Research Portal does not infringe any persons rights or applicable UK laws If you discover content in theResearch Portal that you believe breaches copyright or violates any law please contact openaccessqubacuk
Download date07 Sep 2021
Research ArticleMiniaturization of UWB Antennas on Organic Material
Symeon Nikolaou and Muhammad Ali Babar Abbasi
Department of Electrical Engineering Frederick University 7 Y Frederickou Street Pallouriotisa 1036 Nicosia Cyprus
Correspondence should be addressed to Symeon Nikolaou snikolaoufrederickaccy
Received 16 November 2015 Accepted 9 February 2016
Academic Editor Herve Aubert
Copyright copy 2016 S Nikolaou and M A B Abbasi This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited
Three planar CPW-fed UWB antennas with increasingly reduced size are presented and the miniaturization method is discussedThe first antenna is a CPW-fed elliptical slot with an unevenU-shaped tuning stub the second antenna is a cactus shapedmonopoleand the third one is a miniaturized version of the cactus shaped monopole antenna All presented antennas have a simulatedand measured return loss below minus10 dB over the 31 to 106GHz UWB frequency range and mostly omnidirectional radiationpatterns The proposed antennas are fabricated on liquid crystal polymer (LCP) The CPW-fed slot antenna requires an overallboard dimension of 38mm times 40mm and the evolved cactus monopole is confined in a 28mm times 32mm board while the finalminiaturized cactus monopole is printed on 28mm times 20mm board resulting in a 41 and 63 size reduction respectively Usingboth simulations and measurements the paper analyzes the response of all three antennas and discusses and demonstrates theeffectiveness of the implemented miniaturization method
1 Introduction
The ultrawideband protocol that covers the frequency rangefrom 31 to 106GHz was released from the FCC in 2002[1] partly as an attempt to meet the demand for highdata rate communications in short distances for mobile andpersonal applications Consequently there is an increasingneed for compact sized low cost and high efficiency antennaswith omnidirectional radiation patterns The combinationof these characteristics in a wide frequency range such asthe UWB band is a challenging problem and several designconcepts and differentmaterials have been used in an attemptto provide a satisfactory solution The challenge associatedwith the miniaturization of antennas has been the tradeoffbetween the reduction of the physical size of the antennaand its operational bandwidth [2] and radiation efficiencySome researchers have adopted the use of substrates withrelatively high dielectric constant to reduce the antennarsquosresonant frequency because permittivity is inversely relatedto the resonant frequency [3 4] However bandwidth reduc-tion is inevitable in this approach because bandwidth isalso inversely proportional to the permittivity [2] In anattempt to increase the bandwidth of an antenna and cover
the whole UWB frequency range configurations like planarmonopoles [5ndash8] slot fed IF (inverted F) [6] CPW-fedmonopole [7 9ndash12] U-shaped elliptical slot [13] patch arraywith energy band gap (EBG) structures [3] and CPW-fedfractal antenna [14] have been proposed Also the effectsof printed UWB antenna miniaturization on transmittedtime domain pulse fidelity and pattern stability have beenrecently discussed in [15] In several papers [2 4 16 17]it has been demonstrated that by following certain minia-turization guidelines compact planar UWB antennas canbe made Numerous planar UWB antennas and band-notchUWBquasi-monopole antennas have illustrated symmetricalshapes [12 15 18] In these structures two symmetricalhalves exhibit two identical strong current paths It has beennoted that the current distribution patterns in these struc-tures replicate the conditions of magnetic mirror symmetryChopping off half of the symmetrical monopole antennaprovides straightforward miniaturization It has also beenexplored that by having one strong current path antennaexhibits an even wider bandwidth [15 18 19] however notall half-cut UWB structures can achieve suitable impedancematching over a wide bandwidth A further modificationto the feeding structure is required for better impedance
Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2016 Article ID 5949254 12 pageshttpdxdoiorg10115520165949254
2 International Journal of Antennas and Propagation
matching [19] Increase in cross-polarization level is reportedto be amajor limitation in thisminiaturization technique [18]Another configuration of self-complementary antenna (SCA)has stimulated considerable attention for low profile UWBapplications Theoretically a wideband operation is obtainedwhen an antenna and its complement are identical providingconstant input impedance independent of the antenna geom-etry and frequency Some recent efforts have been devotedto the use of SCA antenna for UWB application Antennasformed by a self-complementing semicircularmonopole [20ndash22] quarter-circular monopole [23 24] obtuse pie-shapedmonopole [25] compact bow-tie [26] and Γ-shaped radiator[27] have been proposed and studied In addition to theimpressive theoretical prospective of SCAs it has to bereported that amatching network is always required tomatchthe antenna to a 50-ohm feed line This has limited the useof SCAs for miniaturized UWB antennas since the matchingnetwork is indispensable In this paper a simple procedurein aggregation with previously proposed guidelines is used tominiaturize an elliptical slot UWB planar antennaThe initialdesign is similar to the one proposed in [13] whereas thefinal novel design is a CPW-fed cactus shaped UWB antennahaving dimensions 20mm times 28mm It is worth mentioningthat the final miniaturized UWB cactus antenna is rathercompact in size and exhibits wider bandwidth as comparedto similar structures presented in [28ndash31] while fabricated onlower 120576
119903material (LCP)
2 Antenna Design and Fabrication
21 Fabrication The proposed antennas are presented inFigure 1 They are fabricated on low loss (tan 120575 = 0002) lowdielectric constant (120576
119903= 3) LCP with a copper layer that
is 18 120583m thick The CPW-fed slot antenna is fabricated on a350 120583m thick substrate while for the cactus antenna and theminiaturized cactus antenna a thinner 225120583m thick substratewas used At the early stages of the design procedure itwas observed that the 350 120583m thick paper substrate exhibitsrigidness To make the cactus antenna conformal as well as
miniaturized substrate thickness was reduced By specifyingthe angle and rate of LCP extrusion while manufacturingthe coefficient of thermal expansion (CTE) can be controlledWith this unique characteristic one can engineer the thermalexpansion of LCP to match with many commonly usedcladding materials like silver copper and so forth [32] Stan-dard photolithography was used for the fabrication The sizereduction of the cactus antenna and the miniaturized cactusis obvious from Figure 1 where the fabricated prototypes arepresented and compared in size with a coin
22 Schematic Discussion The schematics of the comparedantennas are presented in Figure 2 and the dimensionsare summarized in Tables 1 2 and 3 Three antennas arepresented which are the successive evolutions of the firstantenna namely CPW-fed slot antennaThe second antennacalled cactus antenna is a CPW-fedmonopole UWB antennawith 41 (28 times 32mm2 compared to 38 times 40mm2) sizereduction and finally the third version called miniaturizedcactus is a monopole UWB antenna with even smaller RFground patches which is 63 (28 times 20mm2 compared to38 times 40mm2) smaller than the original slot antenna Full-wave EM simulators were used for the design of the presentedprototypes and for the radiation pattern and the return lossoptimization All three antennas are well matched as can beseen from 119878
11plots presented in Figure 3
For the CPW-fed slot the stub dimensions and the lineartapering affect the matching while the ellipse axes size has asmall effect on the radiation patterns The proposed antennais fed by a CPW line with an inner conductor width 119882of 22mm and a gap 119892 between the ground and the innerconductor of 03mm At a distance 119878 = 99mm from theboard edge the inner conductor is linearly tapered until itswidth becomes 09mm to improve the matching betweenthe transmission line and the U-shaped stub The U-shapedstub consists of a semiannular ring and two linear segmentsThe semiannular one has an outer radius 119877 = 55mm andinner radius 119903 = 25mm The left linear segment has length
International Journal of Antennas and Propagation 3
x
yz
D2
C
O
r
R
g
W
S
T
S2
dd
S1L2
D1
L1
(a)W
y z
x
d
D9984001
r
R
g
d2d1
L1
W1
S
D9984002
L3
L2
W3
W2
Gw
Gl
(b)
y z
x
d
D9984001
r
R
g W
d2
d1
L1
W1
S
D9984002
L3
L2W3
W2
Gw
Gl
(c)
Figure 2 Schematic of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
1198781 = 5mm and width 119889 = 3mm while the right linearsegment has length 6mm and width 119889 = 3mm The center119862 of the semiannular ring is 41mm from the ellipse center119874 and the ellipse center is 22mm from the bottom edge Theellipse has a major axis equal to 1198711 = 30mm and secondaryaxis equal to 1198712 = 20mm Overall board dimensions are40mm times 38mm
The evolution of the cactus antenna was based on theobservation that most of the radiated energy for the CPW-fed slot antenna was confined on the tuning stub Thereforea design was attempted without the elliptical slot Impedancematching over the entire UWB range was not satisfactorywith only the two linear segments of the U-shaped tuningstub and to overcome this problem a third linear segment
was added in the feed line direction The thinner LCPsubstrate used for the cactus antenna and the addition of themiddle linear segment required the linearly tapered transitionand the semiannular segment reoptimization Consequentlyfor the cactus antenna the CPW center conductor width119882 is 178mm and length 1198892 is 792mm A linear taperis used to reduce the center conductor width to 119889 =061mm and is connected to the cactus shaped stub atdistance 1198891 = 1024mm from the board edge The tworectangular ground patches have dimensions Gl times Gw whichcorrespond to 944mm and 1489mm respectively For theprimary radiator a cactus shaped stub is used It consists of asemiannular ring with inner radius 119903 = 260mmand an outerradius 119877 = 572mm and three linear segments of different
4 International Journal of Antennas and Propagation
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0S 1
1(d
B)
4 6 8 10 122Frequency (GHz)
(a)
MeasurementSimulation
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(b)
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(c)
Figure 3 Comparison of simulated andmeasured 11987811for (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
lengths The middle linear segment is 1198712 = 1300mm longand 1198822 = 208mm wide while the left and right segmentsare 1198711 = 728mm and 1198713= 156mm long respectivelyBoth of them are 312mm wide From the bottom part of thesemiannular ring a circular sector is detached leaving a chordof length 119878 = 273mm
The third and final evolution of UWB antenna theminiaturized cactus is based primarily on the intermediatedesign and the objective was to decrease the size of the twoground patches Careful design allowed the decrease of theground patches to an overall size of 9mm times 8mm whichcorrespond to Gl and Gw respectively The description ofthe miniaturized cactus design is similar to the presenteddescription for the cactus antenna As a result of the groundsize reduction further tuning was needed for the three stubs
that consist of the cactus shaped radiating element and thedetailed dimensions are summarized in Table 3
International Journal of Antennas and Propagation 5
100e0
125e1
250e2
J sur
f(A
m)
(a)
100e0
125e1
250e2
J sur
f(A
m)
(b)
100e0
125e1
250e2
J sur
f(A
m)
(c)
100e0
125e1
250e2
J sur
f(A
m)
(d)
100e0
125e1
250e2
J sur
f(A
m)
(e)
100e0
125e1
250e2
J sur
f(A
m)
(f)
Figure 4 Surface current (119869) distributions on CPW-fed slot antenna at (a) 5GHz and (b) 9GHz cactus antenna at (c) 5GHz and (d) 9GHzand miniaturized cactus antenna at (e) 5GHz and (f) 9GHz
The overall board dimensions for the cactus antennaare 32mm times 28mm resulting in a 41 reduction in areacompared to the CPW-fed slot while the miniaturized cactushas overall board dimensions of 28mm times 20mm resultingin 63 size reduction compared to the slot antenna Forthe summarized antenna dimensions in Tables 1 2 and 3the common variablesrsquo names are set independently for eachantenna schematic and must not be related
3 Miniaturization
31 Surface Current Distribution The size reduction wasenvisioned by the investigation of the surface current dis-tribution on the slot antenna It was observed that theradiation was primarily caused by the current distributionon the U-shaped stub (Figures 4(a) and 4(b)) although theelliptical slot also contributes to a lesser extent The surface
6 International Journal of Antennas and Propagation
current distributions on the two cactus antennas (Figures4(c)ndash4(f)) have a similar form to the one on the U-shapedstub something that explains the similarity in the resulting
radiation patterns Based on the surface current distributionobservations and trying to improve the matching the evenU-shaped stub (Ant1 from Figure 5) was replaced with an
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
Research ArticleMiniaturization of UWB Antennas on Organic Material
Symeon Nikolaou and Muhammad Ali Babar Abbasi
Department of Electrical Engineering Frederick University 7 Y Frederickou Street Pallouriotisa 1036 Nicosia Cyprus
Correspondence should be addressed to Symeon Nikolaou snikolaoufrederickaccy
Received 16 November 2015 Accepted 9 February 2016
Academic Editor Herve Aubert
Copyright copy 2016 S Nikolaou and M A B Abbasi This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited
Three planar CPW-fed UWB antennas with increasingly reduced size are presented and the miniaturization method is discussedThe first antenna is a CPW-fed elliptical slot with an unevenU-shaped tuning stub the second antenna is a cactus shapedmonopoleand the third one is a miniaturized version of the cactus shaped monopole antenna All presented antennas have a simulatedand measured return loss below minus10 dB over the 31 to 106GHz UWB frequency range and mostly omnidirectional radiationpatterns The proposed antennas are fabricated on liquid crystal polymer (LCP) The CPW-fed slot antenna requires an overallboard dimension of 38mm times 40mm and the evolved cactus monopole is confined in a 28mm times 32mm board while the finalminiaturized cactus monopole is printed on 28mm times 20mm board resulting in a 41 and 63 size reduction respectively Usingboth simulations and measurements the paper analyzes the response of all three antennas and discusses and demonstrates theeffectiveness of the implemented miniaturization method
1 Introduction
The ultrawideband protocol that covers the frequency rangefrom 31 to 106GHz was released from the FCC in 2002[1] partly as an attempt to meet the demand for highdata rate communications in short distances for mobile andpersonal applications Consequently there is an increasingneed for compact sized low cost and high efficiency antennaswith omnidirectional radiation patterns The combinationof these characteristics in a wide frequency range such asthe UWB band is a challenging problem and several designconcepts and differentmaterials have been used in an attemptto provide a satisfactory solution The challenge associatedwith the miniaturization of antennas has been the tradeoffbetween the reduction of the physical size of the antennaand its operational bandwidth [2] and radiation efficiencySome researchers have adopted the use of substrates withrelatively high dielectric constant to reduce the antennarsquosresonant frequency because permittivity is inversely relatedto the resonant frequency [3 4] However bandwidth reduc-tion is inevitable in this approach because bandwidth isalso inversely proportional to the permittivity [2] In anattempt to increase the bandwidth of an antenna and cover
the whole UWB frequency range configurations like planarmonopoles [5ndash8] slot fed IF (inverted F) [6] CPW-fedmonopole [7 9ndash12] U-shaped elliptical slot [13] patch arraywith energy band gap (EBG) structures [3] and CPW-fedfractal antenna [14] have been proposed Also the effectsof printed UWB antenna miniaturization on transmittedtime domain pulse fidelity and pattern stability have beenrecently discussed in [15] In several papers [2 4 16 17]it has been demonstrated that by following certain minia-turization guidelines compact planar UWB antennas canbe made Numerous planar UWB antennas and band-notchUWBquasi-monopole antennas have illustrated symmetricalshapes [12 15 18] In these structures two symmetricalhalves exhibit two identical strong current paths It has beennoted that the current distribution patterns in these struc-tures replicate the conditions of magnetic mirror symmetryChopping off half of the symmetrical monopole antennaprovides straightforward miniaturization It has also beenexplored that by having one strong current path antennaexhibits an even wider bandwidth [15 18 19] however notall half-cut UWB structures can achieve suitable impedancematching over a wide bandwidth A further modificationto the feeding structure is required for better impedance
Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2016 Article ID 5949254 12 pageshttpdxdoiorg10115520165949254
2 International Journal of Antennas and Propagation
matching [19] Increase in cross-polarization level is reportedto be amajor limitation in thisminiaturization technique [18]Another configuration of self-complementary antenna (SCA)has stimulated considerable attention for low profile UWBapplications Theoretically a wideband operation is obtainedwhen an antenna and its complement are identical providingconstant input impedance independent of the antenna geom-etry and frequency Some recent efforts have been devotedto the use of SCA antenna for UWB application Antennasformed by a self-complementing semicircularmonopole [20ndash22] quarter-circular monopole [23 24] obtuse pie-shapedmonopole [25] compact bow-tie [26] and Γ-shaped radiator[27] have been proposed and studied In addition to theimpressive theoretical prospective of SCAs it has to bereported that amatching network is always required tomatchthe antenna to a 50-ohm feed line This has limited the useof SCAs for miniaturized UWB antennas since the matchingnetwork is indispensable In this paper a simple procedurein aggregation with previously proposed guidelines is used tominiaturize an elliptical slot UWB planar antennaThe initialdesign is similar to the one proposed in [13] whereas thefinal novel design is a CPW-fed cactus shaped UWB antennahaving dimensions 20mm times 28mm It is worth mentioningthat the final miniaturized UWB cactus antenna is rathercompact in size and exhibits wider bandwidth as comparedto similar structures presented in [28ndash31] while fabricated onlower 120576
119903material (LCP)
2 Antenna Design and Fabrication
21 Fabrication The proposed antennas are presented inFigure 1 They are fabricated on low loss (tan 120575 = 0002) lowdielectric constant (120576
119903= 3) LCP with a copper layer that
is 18 120583m thick The CPW-fed slot antenna is fabricated on a350 120583m thick substrate while for the cactus antenna and theminiaturized cactus antenna a thinner 225120583m thick substratewas used At the early stages of the design procedure itwas observed that the 350 120583m thick paper substrate exhibitsrigidness To make the cactus antenna conformal as well as
miniaturized substrate thickness was reduced By specifyingthe angle and rate of LCP extrusion while manufacturingthe coefficient of thermal expansion (CTE) can be controlledWith this unique characteristic one can engineer the thermalexpansion of LCP to match with many commonly usedcladding materials like silver copper and so forth [32] Stan-dard photolithography was used for the fabrication The sizereduction of the cactus antenna and the miniaturized cactusis obvious from Figure 1 where the fabricated prototypes arepresented and compared in size with a coin
22 Schematic Discussion The schematics of the comparedantennas are presented in Figure 2 and the dimensionsare summarized in Tables 1 2 and 3 Three antennas arepresented which are the successive evolutions of the firstantenna namely CPW-fed slot antennaThe second antennacalled cactus antenna is a CPW-fedmonopole UWB antennawith 41 (28 times 32mm2 compared to 38 times 40mm2) sizereduction and finally the third version called miniaturizedcactus is a monopole UWB antenna with even smaller RFground patches which is 63 (28 times 20mm2 compared to38 times 40mm2) smaller than the original slot antenna Full-wave EM simulators were used for the design of the presentedprototypes and for the radiation pattern and the return lossoptimization All three antennas are well matched as can beseen from 119878
11plots presented in Figure 3
For the CPW-fed slot the stub dimensions and the lineartapering affect the matching while the ellipse axes size has asmall effect on the radiation patterns The proposed antennais fed by a CPW line with an inner conductor width 119882of 22mm and a gap 119892 between the ground and the innerconductor of 03mm At a distance 119878 = 99mm from theboard edge the inner conductor is linearly tapered until itswidth becomes 09mm to improve the matching betweenthe transmission line and the U-shaped stub The U-shapedstub consists of a semiannular ring and two linear segmentsThe semiannular one has an outer radius 119877 = 55mm andinner radius 119903 = 25mm The left linear segment has length
International Journal of Antennas and Propagation 3
x
yz
D2
C
O
r
R
g
W
S
T
S2
dd
S1L2
D1
L1
(a)W
y z
x
d
D9984001
r
R
g
d2d1
L1
W1
S
D9984002
L3
L2
W3
W2
Gw
Gl
(b)
y z
x
d
D9984001
r
R
g W
d2
d1
L1
W1
S
D9984002
L3
L2W3
W2
Gw
Gl
(c)
Figure 2 Schematic of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
1198781 = 5mm and width 119889 = 3mm while the right linearsegment has length 6mm and width 119889 = 3mm The center119862 of the semiannular ring is 41mm from the ellipse center119874 and the ellipse center is 22mm from the bottom edge Theellipse has a major axis equal to 1198711 = 30mm and secondaryaxis equal to 1198712 = 20mm Overall board dimensions are40mm times 38mm
The evolution of the cactus antenna was based on theobservation that most of the radiated energy for the CPW-fed slot antenna was confined on the tuning stub Thereforea design was attempted without the elliptical slot Impedancematching over the entire UWB range was not satisfactorywith only the two linear segments of the U-shaped tuningstub and to overcome this problem a third linear segment
was added in the feed line direction The thinner LCPsubstrate used for the cactus antenna and the addition of themiddle linear segment required the linearly tapered transitionand the semiannular segment reoptimization Consequentlyfor the cactus antenna the CPW center conductor width119882 is 178mm and length 1198892 is 792mm A linear taperis used to reduce the center conductor width to 119889 =061mm and is connected to the cactus shaped stub atdistance 1198891 = 1024mm from the board edge The tworectangular ground patches have dimensions Gl times Gw whichcorrespond to 944mm and 1489mm respectively For theprimary radiator a cactus shaped stub is used It consists of asemiannular ring with inner radius 119903 = 260mmand an outerradius 119877 = 572mm and three linear segments of different
4 International Journal of Antennas and Propagation
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0S 1
1(d
B)
4 6 8 10 122Frequency (GHz)
(a)
MeasurementSimulation
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(b)
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(c)
Figure 3 Comparison of simulated andmeasured 11987811for (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
lengths The middle linear segment is 1198712 = 1300mm longand 1198822 = 208mm wide while the left and right segmentsare 1198711 = 728mm and 1198713= 156mm long respectivelyBoth of them are 312mm wide From the bottom part of thesemiannular ring a circular sector is detached leaving a chordof length 119878 = 273mm
The third and final evolution of UWB antenna theminiaturized cactus is based primarily on the intermediatedesign and the objective was to decrease the size of the twoground patches Careful design allowed the decrease of theground patches to an overall size of 9mm times 8mm whichcorrespond to Gl and Gw respectively The description ofthe miniaturized cactus design is similar to the presenteddescription for the cactus antenna As a result of the groundsize reduction further tuning was needed for the three stubs
that consist of the cactus shaped radiating element and thedetailed dimensions are summarized in Table 3
International Journal of Antennas and Propagation 5
100e0
125e1
250e2
J sur
f(A
m)
(a)
100e0
125e1
250e2
J sur
f(A
m)
(b)
100e0
125e1
250e2
J sur
f(A
m)
(c)
100e0
125e1
250e2
J sur
f(A
m)
(d)
100e0
125e1
250e2
J sur
f(A
m)
(e)
100e0
125e1
250e2
J sur
f(A
m)
(f)
Figure 4 Surface current (119869) distributions on CPW-fed slot antenna at (a) 5GHz and (b) 9GHz cactus antenna at (c) 5GHz and (d) 9GHzand miniaturized cactus antenna at (e) 5GHz and (f) 9GHz
The overall board dimensions for the cactus antennaare 32mm times 28mm resulting in a 41 reduction in areacompared to the CPW-fed slot while the miniaturized cactushas overall board dimensions of 28mm times 20mm resultingin 63 size reduction compared to the slot antenna Forthe summarized antenna dimensions in Tables 1 2 and 3the common variablesrsquo names are set independently for eachantenna schematic and must not be related
3 Miniaturization
31 Surface Current Distribution The size reduction wasenvisioned by the investigation of the surface current dis-tribution on the slot antenna It was observed that theradiation was primarily caused by the current distributionon the U-shaped stub (Figures 4(a) and 4(b)) although theelliptical slot also contributes to a lesser extent The surface
6 International Journal of Antennas and Propagation
current distributions on the two cactus antennas (Figures4(c)ndash4(f)) have a similar form to the one on the U-shapedstub something that explains the similarity in the resulting
radiation patterns Based on the surface current distributionobservations and trying to improve the matching the evenU-shaped stub (Ant1 from Figure 5) was replaced with an
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
matching [19] Increase in cross-polarization level is reportedto be amajor limitation in thisminiaturization technique [18]Another configuration of self-complementary antenna (SCA)has stimulated considerable attention for low profile UWBapplications Theoretically a wideband operation is obtainedwhen an antenna and its complement are identical providingconstant input impedance independent of the antenna geom-etry and frequency Some recent efforts have been devotedto the use of SCA antenna for UWB application Antennasformed by a self-complementing semicircularmonopole [20ndash22] quarter-circular monopole [23 24] obtuse pie-shapedmonopole [25] compact bow-tie [26] and Γ-shaped radiator[27] have been proposed and studied In addition to theimpressive theoretical prospective of SCAs it has to bereported that amatching network is always required tomatchthe antenna to a 50-ohm feed line This has limited the useof SCAs for miniaturized UWB antennas since the matchingnetwork is indispensable In this paper a simple procedurein aggregation with previously proposed guidelines is used tominiaturize an elliptical slot UWB planar antennaThe initialdesign is similar to the one proposed in [13] whereas thefinal novel design is a CPW-fed cactus shaped UWB antennahaving dimensions 20mm times 28mm It is worth mentioningthat the final miniaturized UWB cactus antenna is rathercompact in size and exhibits wider bandwidth as comparedto similar structures presented in [28ndash31] while fabricated onlower 120576
119903material (LCP)
2 Antenna Design and Fabrication
21 Fabrication The proposed antennas are presented inFigure 1 They are fabricated on low loss (tan 120575 = 0002) lowdielectric constant (120576
119903= 3) LCP with a copper layer that
is 18 120583m thick The CPW-fed slot antenna is fabricated on a350 120583m thick substrate while for the cactus antenna and theminiaturized cactus antenna a thinner 225120583m thick substratewas used At the early stages of the design procedure itwas observed that the 350 120583m thick paper substrate exhibitsrigidness To make the cactus antenna conformal as well as
miniaturized substrate thickness was reduced By specifyingthe angle and rate of LCP extrusion while manufacturingthe coefficient of thermal expansion (CTE) can be controlledWith this unique characteristic one can engineer the thermalexpansion of LCP to match with many commonly usedcladding materials like silver copper and so forth [32] Stan-dard photolithography was used for the fabrication The sizereduction of the cactus antenna and the miniaturized cactusis obvious from Figure 1 where the fabricated prototypes arepresented and compared in size with a coin
22 Schematic Discussion The schematics of the comparedantennas are presented in Figure 2 and the dimensionsare summarized in Tables 1 2 and 3 Three antennas arepresented which are the successive evolutions of the firstantenna namely CPW-fed slot antennaThe second antennacalled cactus antenna is a CPW-fedmonopole UWB antennawith 41 (28 times 32mm2 compared to 38 times 40mm2) sizereduction and finally the third version called miniaturizedcactus is a monopole UWB antenna with even smaller RFground patches which is 63 (28 times 20mm2 compared to38 times 40mm2) smaller than the original slot antenna Full-wave EM simulators were used for the design of the presentedprototypes and for the radiation pattern and the return lossoptimization All three antennas are well matched as can beseen from 119878
11plots presented in Figure 3
For the CPW-fed slot the stub dimensions and the lineartapering affect the matching while the ellipse axes size has asmall effect on the radiation patterns The proposed antennais fed by a CPW line with an inner conductor width 119882of 22mm and a gap 119892 between the ground and the innerconductor of 03mm At a distance 119878 = 99mm from theboard edge the inner conductor is linearly tapered until itswidth becomes 09mm to improve the matching betweenthe transmission line and the U-shaped stub The U-shapedstub consists of a semiannular ring and two linear segmentsThe semiannular one has an outer radius 119877 = 55mm andinner radius 119903 = 25mm The left linear segment has length
International Journal of Antennas and Propagation 3
x
yz
D2
C
O
r
R
g
W
S
T
S2
dd
S1L2
D1
L1
(a)W
y z
x
d
D9984001
r
R
g
d2d1
L1
W1
S
D9984002
L3
L2
W3
W2
Gw
Gl
(b)
y z
x
d
D9984001
r
R
g W
d2
d1
L1
W1
S
D9984002
L3
L2W3
W2
Gw
Gl
(c)
Figure 2 Schematic of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
1198781 = 5mm and width 119889 = 3mm while the right linearsegment has length 6mm and width 119889 = 3mm The center119862 of the semiannular ring is 41mm from the ellipse center119874 and the ellipse center is 22mm from the bottom edge Theellipse has a major axis equal to 1198711 = 30mm and secondaryaxis equal to 1198712 = 20mm Overall board dimensions are40mm times 38mm
The evolution of the cactus antenna was based on theobservation that most of the radiated energy for the CPW-fed slot antenna was confined on the tuning stub Thereforea design was attempted without the elliptical slot Impedancematching over the entire UWB range was not satisfactorywith only the two linear segments of the U-shaped tuningstub and to overcome this problem a third linear segment
was added in the feed line direction The thinner LCPsubstrate used for the cactus antenna and the addition of themiddle linear segment required the linearly tapered transitionand the semiannular segment reoptimization Consequentlyfor the cactus antenna the CPW center conductor width119882 is 178mm and length 1198892 is 792mm A linear taperis used to reduce the center conductor width to 119889 =061mm and is connected to the cactus shaped stub atdistance 1198891 = 1024mm from the board edge The tworectangular ground patches have dimensions Gl times Gw whichcorrespond to 944mm and 1489mm respectively For theprimary radiator a cactus shaped stub is used It consists of asemiannular ring with inner radius 119903 = 260mmand an outerradius 119877 = 572mm and three linear segments of different
4 International Journal of Antennas and Propagation
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0S 1
1(d
B)
4 6 8 10 122Frequency (GHz)
(a)
MeasurementSimulation
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(b)
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(c)
Figure 3 Comparison of simulated andmeasured 11987811for (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
lengths The middle linear segment is 1198712 = 1300mm longand 1198822 = 208mm wide while the left and right segmentsare 1198711 = 728mm and 1198713= 156mm long respectivelyBoth of them are 312mm wide From the bottom part of thesemiannular ring a circular sector is detached leaving a chordof length 119878 = 273mm
The third and final evolution of UWB antenna theminiaturized cactus is based primarily on the intermediatedesign and the objective was to decrease the size of the twoground patches Careful design allowed the decrease of theground patches to an overall size of 9mm times 8mm whichcorrespond to Gl and Gw respectively The description ofthe miniaturized cactus design is similar to the presenteddescription for the cactus antenna As a result of the groundsize reduction further tuning was needed for the three stubs
that consist of the cactus shaped radiating element and thedetailed dimensions are summarized in Table 3
International Journal of Antennas and Propagation 5
100e0
125e1
250e2
J sur
f(A
m)
(a)
100e0
125e1
250e2
J sur
f(A
m)
(b)
100e0
125e1
250e2
J sur
f(A
m)
(c)
100e0
125e1
250e2
J sur
f(A
m)
(d)
100e0
125e1
250e2
J sur
f(A
m)
(e)
100e0
125e1
250e2
J sur
f(A
m)
(f)
Figure 4 Surface current (119869) distributions on CPW-fed slot antenna at (a) 5GHz and (b) 9GHz cactus antenna at (c) 5GHz and (d) 9GHzand miniaturized cactus antenna at (e) 5GHz and (f) 9GHz
The overall board dimensions for the cactus antennaare 32mm times 28mm resulting in a 41 reduction in areacompared to the CPW-fed slot while the miniaturized cactushas overall board dimensions of 28mm times 20mm resultingin 63 size reduction compared to the slot antenna Forthe summarized antenna dimensions in Tables 1 2 and 3the common variablesrsquo names are set independently for eachantenna schematic and must not be related
3 Miniaturization
31 Surface Current Distribution The size reduction wasenvisioned by the investigation of the surface current dis-tribution on the slot antenna It was observed that theradiation was primarily caused by the current distributionon the U-shaped stub (Figures 4(a) and 4(b)) although theelliptical slot also contributes to a lesser extent The surface
6 International Journal of Antennas and Propagation
current distributions on the two cactus antennas (Figures4(c)ndash4(f)) have a similar form to the one on the U-shapedstub something that explains the similarity in the resulting
radiation patterns Based on the surface current distributionobservations and trying to improve the matching the evenU-shaped stub (Ant1 from Figure 5) was replaced with an
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
International Journal of Antennas and Propagation 3
x
yz
D2
C
O
r
R
g
W
S
T
S2
dd
S1L2
D1
L1
(a)W
y z
x
d
D9984001
r
R
g
d2d1
L1
W1
S
D9984002
L3
L2
W3
W2
Gw
Gl
(b)
y z
x
d
D9984001
r
R
g W
d2
d1
L1
W1
S
D9984002
L3
L2W3
W2
Gw
Gl
(c)
Figure 2 Schematic of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
1198781 = 5mm and width 119889 = 3mm while the right linearsegment has length 6mm and width 119889 = 3mm The center119862 of the semiannular ring is 41mm from the ellipse center119874 and the ellipse center is 22mm from the bottom edge Theellipse has a major axis equal to 1198711 = 30mm and secondaryaxis equal to 1198712 = 20mm Overall board dimensions are40mm times 38mm
The evolution of the cactus antenna was based on theobservation that most of the radiated energy for the CPW-fed slot antenna was confined on the tuning stub Thereforea design was attempted without the elliptical slot Impedancematching over the entire UWB range was not satisfactorywith only the two linear segments of the U-shaped tuningstub and to overcome this problem a third linear segment
was added in the feed line direction The thinner LCPsubstrate used for the cactus antenna and the addition of themiddle linear segment required the linearly tapered transitionand the semiannular segment reoptimization Consequentlyfor the cactus antenna the CPW center conductor width119882 is 178mm and length 1198892 is 792mm A linear taperis used to reduce the center conductor width to 119889 =061mm and is connected to the cactus shaped stub atdistance 1198891 = 1024mm from the board edge The tworectangular ground patches have dimensions Gl times Gw whichcorrespond to 944mm and 1489mm respectively For theprimary radiator a cactus shaped stub is used It consists of asemiannular ring with inner radius 119903 = 260mmand an outerradius 119877 = 572mm and three linear segments of different
4 International Journal of Antennas and Propagation
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0S 1
1(d
B)
4 6 8 10 122Frequency (GHz)
(a)
MeasurementSimulation
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(b)
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(c)
Figure 3 Comparison of simulated andmeasured 11987811for (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
lengths The middle linear segment is 1198712 = 1300mm longand 1198822 = 208mm wide while the left and right segmentsare 1198711 = 728mm and 1198713= 156mm long respectivelyBoth of them are 312mm wide From the bottom part of thesemiannular ring a circular sector is detached leaving a chordof length 119878 = 273mm
The third and final evolution of UWB antenna theminiaturized cactus is based primarily on the intermediatedesign and the objective was to decrease the size of the twoground patches Careful design allowed the decrease of theground patches to an overall size of 9mm times 8mm whichcorrespond to Gl and Gw respectively The description ofthe miniaturized cactus design is similar to the presenteddescription for the cactus antenna As a result of the groundsize reduction further tuning was needed for the three stubs
that consist of the cactus shaped radiating element and thedetailed dimensions are summarized in Table 3
International Journal of Antennas and Propagation 5
100e0
125e1
250e2
J sur
f(A
m)
(a)
100e0
125e1
250e2
J sur
f(A
m)
(b)
100e0
125e1
250e2
J sur
f(A
m)
(c)
100e0
125e1
250e2
J sur
f(A
m)
(d)
100e0
125e1
250e2
J sur
f(A
m)
(e)
100e0
125e1
250e2
J sur
f(A
m)
(f)
Figure 4 Surface current (119869) distributions on CPW-fed slot antenna at (a) 5GHz and (b) 9GHz cactus antenna at (c) 5GHz and (d) 9GHzand miniaturized cactus antenna at (e) 5GHz and (f) 9GHz
The overall board dimensions for the cactus antennaare 32mm times 28mm resulting in a 41 reduction in areacompared to the CPW-fed slot while the miniaturized cactushas overall board dimensions of 28mm times 20mm resultingin 63 size reduction compared to the slot antenna Forthe summarized antenna dimensions in Tables 1 2 and 3the common variablesrsquo names are set independently for eachantenna schematic and must not be related
3 Miniaturization
31 Surface Current Distribution The size reduction wasenvisioned by the investigation of the surface current dis-tribution on the slot antenna It was observed that theradiation was primarily caused by the current distributionon the U-shaped stub (Figures 4(a) and 4(b)) although theelliptical slot also contributes to a lesser extent The surface
6 International Journal of Antennas and Propagation
current distributions on the two cactus antennas (Figures4(c)ndash4(f)) have a similar form to the one on the U-shapedstub something that explains the similarity in the resulting
radiation patterns Based on the surface current distributionobservations and trying to improve the matching the evenU-shaped stub (Ant1 from Figure 5) was replaced with an
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
4 International Journal of Antennas and Propagation
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0S 1
1(d
B)
4 6 8 10 122Frequency (GHz)
(a)
MeasurementSimulation
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(b)
MeasurementSimulation
minus35
minus30
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
(c)
Figure 3 Comparison of simulated andmeasured 11987811for (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactus antenna
lengths The middle linear segment is 1198712 = 1300mm longand 1198822 = 208mm wide while the left and right segmentsare 1198711 = 728mm and 1198713= 156mm long respectivelyBoth of them are 312mm wide From the bottom part of thesemiannular ring a circular sector is detached leaving a chordof length 119878 = 273mm
The third and final evolution of UWB antenna theminiaturized cactus is based primarily on the intermediatedesign and the objective was to decrease the size of the twoground patches Careful design allowed the decrease of theground patches to an overall size of 9mm times 8mm whichcorrespond to Gl and Gw respectively The description ofthe miniaturized cactus design is similar to the presenteddescription for the cactus antenna As a result of the groundsize reduction further tuning was needed for the three stubs
that consist of the cactus shaped radiating element and thedetailed dimensions are summarized in Table 3
International Journal of Antennas and Propagation 5
100e0
125e1
250e2
J sur
f(A
m)
(a)
100e0
125e1
250e2
J sur
f(A
m)
(b)
100e0
125e1
250e2
J sur
f(A
m)
(c)
100e0
125e1
250e2
J sur
f(A
m)
(d)
100e0
125e1
250e2
J sur
f(A
m)
(e)
100e0
125e1
250e2
J sur
f(A
m)
(f)
Figure 4 Surface current (119869) distributions on CPW-fed slot antenna at (a) 5GHz and (b) 9GHz cactus antenna at (c) 5GHz and (d) 9GHzand miniaturized cactus antenna at (e) 5GHz and (f) 9GHz
The overall board dimensions for the cactus antennaare 32mm times 28mm resulting in a 41 reduction in areacompared to the CPW-fed slot while the miniaturized cactushas overall board dimensions of 28mm times 20mm resultingin 63 size reduction compared to the slot antenna Forthe summarized antenna dimensions in Tables 1 2 and 3the common variablesrsquo names are set independently for eachantenna schematic and must not be related
3 Miniaturization
31 Surface Current Distribution The size reduction wasenvisioned by the investigation of the surface current dis-tribution on the slot antenna It was observed that theradiation was primarily caused by the current distributionon the U-shaped stub (Figures 4(a) and 4(b)) although theelliptical slot also contributes to a lesser extent The surface
6 International Journal of Antennas and Propagation
current distributions on the two cactus antennas (Figures4(c)ndash4(f)) have a similar form to the one on the U-shapedstub something that explains the similarity in the resulting
radiation patterns Based on the surface current distributionobservations and trying to improve the matching the evenU-shaped stub (Ant1 from Figure 5) was replaced with an
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
International Journal of Antennas and Propagation 5
100e0
125e1
250e2
J sur
f(A
m)
(a)
100e0
125e1
250e2
J sur
f(A
m)
(b)
100e0
125e1
250e2
J sur
f(A
m)
(c)
100e0
125e1
250e2
J sur
f(A
m)
(d)
100e0
125e1
250e2
J sur
f(A
m)
(e)
100e0
125e1
250e2
J sur
f(A
m)
(f)
Figure 4 Surface current (119869) distributions on CPW-fed slot antenna at (a) 5GHz and (b) 9GHz cactus antenna at (c) 5GHz and (d) 9GHzand miniaturized cactus antenna at (e) 5GHz and (f) 9GHz
The overall board dimensions for the cactus antennaare 32mm times 28mm resulting in a 41 reduction in areacompared to the CPW-fed slot while the miniaturized cactushas overall board dimensions of 28mm times 20mm resultingin 63 size reduction compared to the slot antenna Forthe summarized antenna dimensions in Tables 1 2 and 3the common variablesrsquo names are set independently for eachantenna schematic and must not be related
3 Miniaturization
31 Surface Current Distribution The size reduction wasenvisioned by the investigation of the surface current dis-tribution on the slot antenna It was observed that theradiation was primarily caused by the current distributionon the U-shaped stub (Figures 4(a) and 4(b)) although theelliptical slot also contributes to a lesser extent The surface
6 International Journal of Antennas and Propagation
current distributions on the two cactus antennas (Figures4(c)ndash4(f)) have a similar form to the one on the U-shapedstub something that explains the similarity in the resulting
radiation patterns Based on the surface current distributionobservations and trying to improve the matching the evenU-shaped stub (Ant1 from Figure 5) was replaced with an
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
current distributions on the two cactus antennas (Figures4(c)ndash4(f)) have a similar form to the one on the U-shapedstub something that explains the similarity in the resulting
radiation patterns Based on the surface current distributionobservations and trying to improve the matching the evenU-shaped stub (Ant1 from Figure 5) was replaced with an
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
International Journal of Antennas and Propagation 7
minus25
minus20
minus15
minus10
minus5
0
S 11
(dB)
4 6 8 10 122Frequency (GHz)
L2
L2 = 10mmL2 = 12mmL2 = 14mm
Figure 7 11987811with 1198712 variation
minus40
minus35
minus30
minus25
minus20
minus15
minus10
minus5
S 11
(dB)
4 6 8 10 122Frequency (GHz)
Gw
Gw = 14mmGw = 12mmGw = 10mm
Figure 8 11987811with ground width (Gw) variation
unevenU-shaped stubThe added perturbation on the tuningstub added one design degree of freedom that allowed theimprovement of the matching as can be seen in Figure 6 Theuneven U-shaped slot which is presented in Figure 5 underthe name Ant2 had improved matching as can be seen in 119878
11
plots of Figure 5 In the next iteration (Ant3) the slot wasremoved and in order to further improve the matching forthe remaining U-shaped stub a third tuning stub was addedalong the direction of the feed line resulting in the cactusshaped radiator (Ant4) that evolved eventually after someadditional tuning to the miniaturized cactus antenna Thisthird middle stub allowed for an additional design parameterand as a result of its bigger length the matching in the lowerend of the UWB range in the area around 31 GHz could beimprovedThematching improvement in the lower frequencyend is evident in Figure 5 and the presented frequency notch
that can be seen in Figure 7 (red dotted line) as a result of theadditional third stub can be easily suppressed with the carefulselection of the stub size 1198712
32 Ground Size Reduction The miniaturization process sofar led to the Ant4 structure shown in Figure 5This structurewas further optimized and the final structure is presented ascactus antenna in Figure 1(b) However the overall antennasize could be further improved by attempting a ground patchreduction in addition to the removal of the elliptical slotThe idea was also based on the study of the surface currentdistribution of the cactus antenna (Figures 4(c) and 4(d))where the current intensity along the outer edges of therectangular ground patches is clearly lower than the currentintensity on the edges closer to the signal line Parametricstudy of the width of the ground patches (Gw) showed
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
8 International Journal of Antennas and Propagation
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(f)
Figure 9 Simulated and measured 119864-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
only little effect on 11987811plots (Figure 8) and the optimization
steps resulted in the miniaturized cactus version depicted inFigure 1(c) with Gw equal to only 8mm and overall boarddimensions 20mm times 28mm which is equivalent to 63 sizereduction compared to the original design of the CPW-fedslot antenna
4 Discussion of Measurements andSimulation Results
41 Return Loss For return loss and radiation pattern mea-surements an SMA connector was soldered onto the boardAn HP8530 Network Analyzer was used to measure thereturn loss which is shown in Figure 3 with the simulatedreturn loss For the CPW-fed slot two main resonances areobserved in both the simulated and the measured return lossplots which are controlled by the two linear segments on theU-shaped stub The simulated return loss is well matched
from 3GHz to over 12GHz but the measured return loss isslightly worse thanminus10 dB around 8GHz however it remainsmatched up to the frequency of 106GHz which is the upperbound for the UWB frequency range
The simulated and measured return loss for both cac-tus antenna (Figure 3(b)) and miniaturized cactus antenna(Figure 3(c)) are obviously better especially at the two endsof the frequency range with a better than minus10 dB return lossfrom 29GHz to 12GHz that overlaps the designated UWBrange Three resonances dominate the return loss for thecactus antenna these appear at 37 51 and 64GHz onefor each linear segment Generally the longer the stub isthe lower the corresponding resonance appears to be Thiscan be seen in Figure 7 where the simulated 119878
11is plotted
for three different length values (1198712) of the longest linearsegment The matching at the higher frequencies is affectedby the rectangular ground patchesrsquo width Gw as can be seenin Figure 8 where 119878
11is plotted for three different Gw values
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
International Journal of Antennas and Propagation 9
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(a)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(b)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(c)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(d)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40
minus30
minus20
minus10
(e)
0
0
30
6090
120
150
180
210
240270
300
330
MeasurementSimulation
minus40minus30
minus20
minus10
(f)
Figure 10 Simulated and measured119867-plane radiation patterns of (a) CPW-fed slot antenna (b) cactus antenna and (c) miniaturized cactusantenna at 5GHz and (d) CPW-fed slot antenna (e) cactus antenna and (f) miniaturized cactus antenna at 9GHz
for the miniaturized cactus antenna It was concluded thatthe width of the ground patch cannot be smaller than 8mmwithout compromising matching at higher frequencies andradiation patterns consistency although it would be highlydesired for an even more compact design
For the presented 11987811
plots there is a small discrepancybetween the simulated and measured results This is partlydue to the fact that the UWB range is large compared to thecentral frequency and it is difficult for the frequency domainsimulation tools to give accurate results over the whole bandMoreover the size of the SMA connector which is significantcompared to the size of the antennas causes additionaldiscrepancy between measurements and simulated resultsfor which a CPWmode excitation port was used
42 Radiation Patterns and Gain Measured and simulatedradiation patterns for all three antennas at 5 and 9GHzwhich are representative of the patterns across the frequency
range are presented in Figures 9 and 10 Figure 9 presentsthe 119864-plane (119909-119911) copolarization where 120579 = 0∘ correspondsto the 119911-axis and 120579 = 90∘ corresponds to the 119909-axis It isseen that for all three antenna designs the 119864-plane has anull along the 119909-axis due to the feed line and a pattern thatis nearly symmetric around the 119909-axis The 119867-plane (119910-119911)copolarization plots are presented in Figure 10 where 120579= 0∘ isthe 119911-axis and 120579 = 90∘ is the 119910-axis It is seen that the119867-planepatterns for both cactus antenna designs are almost perfectlyomnidirectional at 5GHz and mostly omnidirectional at9GHz however particularly at 9GHz the slot antenna 119867-plane pattern flattens along horizontal axis This somewhatdirectional behavior is verified by the gain measurementswhich are taken along the 119911-axis direction shown in Figure 11As can be deduced from Figure 11(a) the gain at 5GHz and9GHz for the slot antenna is 5 dBi and 4 dBi respectivelyThe evident discrepancy between simulated and measuredpeak gain shown in Figure 11(a) can be explained by relatively
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
10 International Journal of Antennas and Propagation
MeasurementSimulation
108 976543Frequency (GHz)
minus2
minus1
0
1
2
3
4
5
6
7G
ain
(dBi
)
(a)
MeasurementSimulation
108 976543Frequency (GHz)
minus4
minus3
minus2
minus1
0
1
2
3
4
Gai
n (d
Bi)
(b)
MeasurementSimulation
108 976543Frequency (GHz)
minus15
minus1
minus05
0
05
1
15
2
25
3
35
Gai
n (d
Bi)
(c)
108 976543Frequency (GHz)
minus3
minus2
minus1
0
1
2
3
4
5
6
Gai
n (d
Bi)
CPW-fed slot antennaCactus antenna
Miniaturized cactus antenna
(d)
Figure 11 Simulated andmeasured gain comparison for (a) CPW-fed slot antenna (b) cactus antenna and (c)miniaturization cactus antennaand (d) comparison of measured gain for all three antennas
more directive measured 119864-plane pattern when comparingwith the simulated 119864-plane pattern shown in Figure 9(a) Adirective beamwithmaxima at 37∘ was observed inmeasured119864-plane pattern resulting in a 22 dBi higher peak gainvalue when compared with the simulated predictions Thismore directive measured pattern can be directly related tofabrication anomalies Both cactus shaped antennasmaintainalmost perfectly omnidirectional radiation patterns whichis also verified from the gain plot which is close to 0 dBiParticularly the miniaturized cactus antenna in additionto its compact size presents rather constant gain whichimproves the fidelity of transmitted time domain fast pulses[15] The additional size of the slot antenna as a result of theincluded elliptical slotmakes the antennamore directive andfor some applications this could be an advantage Howeverconsidering that most applications involve mobile handheld
devices omnidirectional characteristics can be an overalladvantage for a UWB antenna
5 Conclusions
Three proposed antennas are fabricated on flexible low lossand low cost LCP organic material and a miniaturizationmethod is discussed All three antennas have a returnloss better than minus10 dB in the whole ultrawideband rangeand have close to omnidirectional radiation patterns Theevolved cactus antenna and miniaturized cactus antenna aredeveloped based on the fact that the original slot antennarsquosoperation depends primarily upon the current distributionon the U-shaped tuning stub Based on this observationregions with relatively lower surface current amplitude wereremoved to achieve more compact size reduced device
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
International Journal of Antennas and Propagation 11
During this process the U-shaped stub elliptical slot antennawas modified to form a cactus shaped radiator The radiationcharacteristics of cactus were thoroughly investigated and thenew antenna was optimized to be well matched in the wholeUWB range The bigger cactus antenna covers only 59 ofthe area of the original CPW-fed slot antenna whereas theminiaturized cactus antenna covers only 37 of the initialarea As a consequence of the removal of the elliptical slot themonopole cactus antennas became more omnidirectionalThe good agreement between simulated andmeasured resultsverifies the good performance of the proposed antennasand validates the success of the proposed miniaturizationmethod
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to thank colleagues from GaTechAthena andMircTech research groups and fromNASAGlennfor their assistance in antennasrsquo testing
References
[1] FCC FCC First Report and Order on Ultra-Wideband Technol-ogy FCC Washington DC USA 2002
[2] A M Abbosh ldquoMiniaturization of planar ultrawideband an-tenna via corrugationrdquo IEEEAntennas andWireless PropagationLetters vol 7 pp 685ndash688 2008
[3] D Nashaat H A Elsadek E A Abdallah M F Iskanderand H M Elhenawy ldquoUltrawide bandwidth 2 times 2 microstrippatch array antenna using electromagnetic band-gap structure(EBG)rdquo IEEE Transactions on Antennas and Propagation vol59 no 5 pp 1528ndash1534 2011
[4] A M Abbosh ldquoMiniaturized microstrip-fed tapered-slotantenna with ultrawideband performancerdquo IEEE Antennas andWireless Propagation Letters vol 8 pp 690ndash692 2009
[5] M Ezuma S Subedi and J-Y Pyun ldquoDesign of a compactUWB antenna formulti-bandwireless applicationsrdquo in Proceed-ings of the International Conference on Information Networking(ICOIN rsquo15) pp 456ndash461 IEEE Jeju Island South KoreaJanuary 2015
[6] C-Y Sim W-T Chung and C-H Lee ldquoCompact slot antennaforUWBapplicationsrdquo IEEEAntennas andWireless PropagationLetters vol 9 pp 63ndash66 2010
[7] M Koohestani N Pires A K Skrivervik and A A MoreiraldquoInfluence of the human body on a new coplanar-fed Ultra-Wideband antennardquo in Proceedings of the 6th European Con-ference on Antennas and Propagation (EuCAP rsquo12) pp 316ndash319Prague Czech Republic March 2012
[8] D N Elsheakh H A Elsadek E A Abdallah M F Iskan-der and H Elhenawy ldquoUltrawide bandwidth umbrella-shapedmicrostrip monopole antenna using spiral artificial magneticconductor (SAMC)rdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1255ndash1258 2009
[9] M Koohestani A A Moreira and A K Skrivervik ldquoAnovel compact CPW-fed polarization diversity ultrawideband
[10] S Nikolaou and G E Ponchak ldquoCompact cactus-shapedUltra Wide-Band (UWB) monopole on organic substraterdquo inProceedings of the IEEE Antennas and Propagation Society Inter-national Symposium pp 4637ndash4640 IEEE Honolulu HawaiiUSA June 2007
[11] J Liang L Guo C C Chiau X Chen and C G Parini ldquoStudyof CPW-fed circular disc monopole antenna for ultra widebandapplications rdquo IEE Microwaves Antennas and PropagationProceedings vol 152 no 6 pp 520ndash526 2005
[12] J Kim T Yoon J Kim and J Choi ldquoDesign of an ultrawide-band printed monopole antenna using FDTD and geneticalgorithmrdquo IEEE Microwave and Wireless Components Lettersvol 15 no 6 pp 395ndash397 2005
[13] P Li J Liang and X Chen ldquoStudy of printed ellipticalcircularslot antennas for ultrawideband applicationsrdquo IEEE Transac-tions on Antennas and Propagation vol 54 no 6 pp 1670ndash16752006
[14] T Sedghi M Jalali and T Aribi ldquoFabrication of CPW-fedfractal antenna for UWB applications with omni-directionalpatternsrdquo The Scientific World Journal vol 2014 Article ID391602 5 pages 2014
[15] J Liu K P Esselle S G Hay and S Zhong ldquoEffects of printedUWB antenna miniaturization on pulse fidelity and patternstabilityrdquo IEEE Transactions on Antennas and Propagation vol62 no 8 pp 3903ndash3910 2014
[16] Z N Chen ldquoMiniaturization of ultra-wideband antennasinvited paperrdquo in Proceedings of the Asia-Pacific MicrowaveConference (APMC rsquo11) pp 1290ndash1293 December 2011
[17] A K Amert andKWWhites ldquoMiniaturization of the biconicalantenna for ultrawideband applicationsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 12 pp 3728ndash3735 2009
[18] M Sun Y P Zhang and Y Lu ldquoMiniaturization of planarmonopole antenna for ultrawideband radiosrdquo IEEE Transac-tions onAntennas and Propagation vol 58 no 7 pp 2420ndash24252010
[19] A Mobashsher and A Abbosh ldquoUtilizing symmetry of planarultra-wideband antennas for size reduction and enhancedperformancerdquo IEEE Antennas and Propagation Magazine vol57 no 2 pp 153ndash166 2015
[20] L Guo S Wang Y Gao Z Wang X Chen and C G ParinildquoStudy of printed quasi-self-complementary antenna for ultra-wideband systemsrdquoElectronics Letters vol 44 no 8 pp 511ndash5122008
[21] L Guo X Chen and C G Parini ldquoMiniature ultra-widebandantenna for wireless universal serial bus dongle applicationsrdquoIET Microwaves Antennas amp Propagation vol 6 no 1 pp 113ndash119 2012
[22] L Guo SWang X Chen and C Parini ldquoA small printed quasi-self-complementary antenna for ultrawideband systemsrdquo IEEEAntennas and Wireless Propagation Letters vol 8 pp 554ndash5572009
[23] C-Y Huang and J-Y Su ldquoA printed band-notched UWBantenna using quasi-self-complementary structurerdquo IEEEAntennas and Wireless Propagation Letters vol 10 pp 1151ndash1153 2011
[24] C-C Lin C-Y Huang and J-Y Su ldquoUltra-wideband quasi-self-complementary antenna with band-rejection capabilityrdquoIET Microwaves Antennas and Propagation vol 5 no 13 pp1613ndash1618 2011
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
12 International Journal of Antennas and Propagation
[25] C-C Lin C-Y Huang and G-H Chen ldquoObtuse pie-shapedquasi-self-complementary antenna for WLAN applicationsrdquoIEEEAntennas andWireless Propagation Letters vol 12 pp 353ndash355 2013
[26] C-C Lin ldquoCompact bow-tie quasi-self-complementaryantenna for UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 11 pp 987ndash989 2012
[27] L Guo S Wang X Chen and C G Parini ldquoStudy of compactantenna for UWB applicationsrdquo Electronics Letters vol 46 no2 pp 115ndash116 2010
[28] C Saephan H Khaleel B Valdovinos A Isaac and A BihnamldquoTri-band cactus shaped printed monopolerdquo in Proceedingsof the IEEE Antennas and Propagation Society InternationalSymposium (APSURSI rsquo14) pp 1704ndash1705 IEEE MemphisTenn USA July 2014
[29] S K Mishra R K Gupta A Vaidya and J MukherjeeldquoA compact dual-band fork-shaped monopole antenna forbluetooth and UWB applicationsrdquo IEEE Antennas and WirelessPropagation Letters vol 10 pp 627ndash630 2011
[30] V Zachou C G Christodoulou M T Chryssomallis DAnagnostou and S Barbin ldquoPlanar monopole antenna withattached sleevesrdquo IEEE Antennas and Wireless PropagationLetters vol 5 no 1 pp 286ndash289 2006
[31] M J Ammann and R Farrell ldquoDual-band monopole antennawith stagger-tuned arms for broadbandingrdquo in Proceedings ofthe IEEE International Workshop on Antenna Technology SmallAntennas and Novel Metamaterials (IWAT rsquo05) pp 278ndash281IEEE March 2005
[32] D C Thompson O Tantot H Jallageas G E Ponchak MM Tentzeris and J Papapolymerou ldquoCharacterization of liquidcrystal polymer (LCP) material and transmission lines on LCPsubstrates from 30 to 110GHzrdquo IEEETransactions onMicrowaveTheory and Techniques vol 52 no 4 pp 1343ndash1352 2004
![[XLS]icccsconf.orgicccsconf.org/ICCCS 2018 Accepted Papers.xlsx · Web viewA high gain, noise cancelling 3.1-10.6 GHz CMOS LNA for UWB application Facebook5K: A novel evaluation resource](https://static.documents.pub/doc/80x56/5ae5a5b57f8b9a3d3b8c2544/xls-2018-accepted-papersxlsxweb-viewa-high-gain-noise-cancelling-31-106-ghz.jpg)
