1Department of Electrical Engineering, College of Engineering, University of Basrah, Basra, Iraq2Department of Electrical Techniques, Qurna Technique institute, Southern Technical University, Basra, Iraq
Received: 10.03.2018 • Accepted/Published Online: 26.08.2018 • Final Version: 22.01.2019
Abstract: This paper presents a compact wide-slot ultrawide-band (UWB) antenna with reconfigurable and sharpdual-band notches compatible with the underlay cognitive radio systems. The antenna slot and tuning stub are reshapedin such a way that the proposed antenna operating bandwidth extends along the frequency band specified for UWBapplications (3.1–10.6 GHz). In spite of their sharpness, the generated band notches perfectly cancel the interferencewith the entire 5 GHz wireless local area network (WLAN) frequency band (5.15–5.725 GHz) and that of the militaryX-band satellite communications downlink (7.25–7.745 GHz). The WLAN band notch is presented by attaching a pair ofquarter wavelength parasitic elements to the antenna ground plane, while the other band notch is produced by engravinganother pair of half wavelength parasitic elements within the antenna wide slot along the circumference of the antennatuning stub. The reconfiguration of the band notches is achieved by inserting a pair of PIN diodes for each pair ofparasitic elements. The simulated and measured results accentuate the UWB coverage of the proposed antenna, andalso verify the reconfigurability and the sharpness of the antenna band notches. Moreover, the antenna has a noticeablystable radiation pattern over the entire band of operation with omnidirectional pattern convenient for portable UWBgadgets.
Key words: Cognitive radio, ultrawide-band, wide-slot antenna, reconfigurable band notch
1. IntroductionDue to the enormous deployment of wireless communication systems, the frequency spectrum allocation haswitnessed a noticeable congestion in the last few decades. This serious issue has restricted the response tothe incredible customers’ demand for new forms of high-performance communication systems. Cognitive radio(CR) systems were invented to mitigate the limitations of the frequency spectrum allocation. As identified bythe Federal Communication Commission (FCC), about 70% of the allocated spectrum is idle for a remarkableamount of time [1]. CR systems recruit the idle frequency, which is specified for a licensed user (primary user),for another unlicensed user (secondary user). In other words, the secondary user can communicate via theprimary user’s channel as long as the primary user is in its idle mode. CR systems are divided into interweaveand underlay systems. The former category of CR systems uses an ultrawide-band (UWB) antenna to sense thepresence of the idle channels and another reconfigurable antenna for communication purposes [2]. On the otherhand, the underlay CR systems, which represent the main scope of this work, utilize a single UWB antenna with∗Correspondence: [email protected]
This work is licensed under a Creative Commons Attribution 4.0 International License.94
reconfigurable band notches to cancel the communication via the licensed channels during the active modes ofthe primary users.
The underlay CR systems transmit a reduced amount of power along the UWB frequency band (3.1–10.6GHz) to minimize the interference with the already existing applications. Some applications do not acceptany amount of interference, so the UWB antenna of the underlay systems should have the capability of bandnotch generation to completely eliminate the interference with these critical applications. Like the conventionalfrequency reconfiguration, the reconfiguration of the antenna band notch may be accomplished with the aid ofradio frequency (RF) switches such as PIN diodes [3], MEMS switches [4], and photoconductive switches [5]. Inaddition, the band notch center frequency can be tuned by attaching a varactor diode to the designed antenna[6]. For planar monopole UWB antennas, reconfigurable band notches can be presented in many ways such ascontrolling the length of a slot etched on the ground plane [7], modifying the length of parasitic elements [8–10],or reshaping split ring resonators [11]. However, the band notch reconfiguration of wide-slot UWB antennas maybe achieved by controlling the length of slots etched on the antenna tuning stubs[12–14], attaching a variablelength parasitic element to the radiating stub [15], or modifying the length of parasitic elements attached tothe tuning stub [16].
A compact wide-slot UWB antenna with two sharp and reconfigurable band notches for underlay CRcommunications is proposed in this paper. The shapes of the slot and the tuning stub of the proposed antennaare modified so that the antenna operating bandwidth covers the entire UWB frequency spectrum (3.1–10.6GHz). The two band notches are generated by presenting two parasitic elements within the antenna slot. Thefirst band notch is generated by inserting a pair of quarter wavelength parasitic elements to the ground planeto reject the radiation of the 5GHz WLAN (5.15–5.725 GHz). A pair of half wavelength parasitic elements isengraved on the ground plane side of the antenna along the circumference of the antenna tuning stub to avoidthe interference with the down link of the military X-band satellite communications (7.25–7.745 GHz). Eachband notch is switched ON and OFF by a pair of PIN diodes. The measurements agree well with the simulatedresults and both verify the sharpness and successful reconfiguration of the two band notches.
2. Antenna designIn general, the planar wide-slot UWB antenna consists of feed line, radiating wide slot etched on the groundplane, and tuning stub to enhance the coupling between the feed line and the antenna radiating slot [17]. Figures1a and 1b illustrate the structure and the dimensions of the proposed wide-slot UWB antenna attached to a50-Ω microstrip feed line. The dielectric substrate of the proposed antenna is Rogers RT5880 whose dielectricconstant and loss tangent are 2.2 and 4 × 10−4 , respectively. The overall dimensions of the antenna are 28 ×26 × 0.8 mm3 .
The parameters L1 and L2 (see Figure 1) modify the shape of the wide-slot, while S and H controls theinset length and the triangular part height of the antenna tuning stub, respectively. L1, L2, S, and H worktogether to produce antenna bandwidth that perfectly covers the frequency band specified for UWB applications(3.1–10.6 GHz). On the other hand, d1 modifies the length of the quarter wavelength parasitic elements thatare responsible for generating the band notch that cancels out the radiation of the 5-GHz WLAN applications.The parameter d2 determines the length of the half wavelength parasitic elements that are presented to providea band notch to reject the radiation of the downlink of the military X-band satellite radiation. It is clear fromFigure 1 that the two half wavelength parasitic elements meet each other at their top vertices to form one fullwavelength parasitic element.
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(a)
28
26
8
H
5.5
8.5
S
2
x
y
Z
(b)
3
L2
L1
4
5
3
0.5
0.5
d1 d1
d2 1
PIN Diode Group 2
PIN Diode Group 1
2
3
8
Figure 1. The structure of the proposed reconfigurable wide-slot antenna (a) front view and (b) back view (all dimensionsare in mm.
The presence and the absence of the band notches are determined by the ON and OFF states of thetwo groups of PIN diodes, respectively. Group 1 is attached to the quarter wavelength parasitic elements,whereas Group 2 is inserted to the half wavelength parasitic elements. All PIN diodes used in this antenna areSMP1320-079LF whose reverse biasing capacitor is (Cr = 0.3 pF), and forward resistance value is (Rf = 0.9 Ω
at 10 mA). It is worth mentioning that the OFF and ON states of the PIN diodes are simulated by the reversebiasing capacitance and the forward biasing resistance, respectively, during the antenna simulation process.
3. Parametric study
The simulation of the proposed antenna was done with the aid of CST Microwave studio simulation suite [18].This section is divided into two subsections, the first one studies the slot shape and tuning stub effects on theUWB characteristic of the antenna while the second one investigates the effect of the parasitic elements on theantenna band notch generation.
3.1. UWB parametric study
In this subsection, the effect of the parasitic elements is neglected by setting both PIN diode groups to theirOFF state. The steps that are followed until obtaining the final UWB antenna structure can be summarized asfollows:
Step 1. Antenna structure with rectangular slot and rectangular tuning stub with the proposed compact sizeis presented.
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Step 2. The upper side of the tuning stub is modified to a triangular shape which improved the coupling withthe slot [19].
Step 3. An inset is engraved in the tuning stub to modify the feed point position which effect the antennabandwidth significantly.
Step 4. The rectangular slot is then modified to cover the entire frequency band of the UWB applications asshown in Figure 1.
Figure 2 illustrates the magnitude of the reflection coefficient for each of the aforementioned steps. Theinfluence of the slot shape is demonstrated in the reflection coefficient ( |S11| ) plot shown in Figures 3a and 3b.The case is started by setting the value of L1 and L2 to zero as shown in Figure 3a. The resulting rectangular slotprovides a bandwidth that does not fit the UWB frequency band. The higher edge of the reflection coefficient isshifted to 11 GHz by increasing the value of L1 up to 4 mm. Unfortunately, the reflection coefficient value withinthe frequency range (7–9.8 GHz) surpasses −10 dB, so L2 is increased to improve the impedance bandwidthof the antenna as shown in Figure 3b. It is clear that at L2 = 4 mm, the antenna bandwidth reaches 8 GHzextended from 3G Hz up to 11 GHz.
2 4 6 8 10 12 2 -50
-
40
-30
- 20
-10
0
10
Frequency (GHz)
|S11
|(d
B)
Step 1 Step 2 Step 3 Step 4
Figure 2. Simulated reflection coefficient for the four design steps of the proposed antenna.
Since the matching stub controls the coupling between the feed line and the radiating slot, the influenceof the stub triangular part height (H) and the length of the stub inset (S) are revealed in Figures 4a and 4b,respectively. H = 6 mm and S = 2 mm are the best values that lead to obtain the aforementioned bandwidth.
3.2. Parametric study for band notches generationBy setting both PIN diode groups to their ON state, the effect of parasitic elements shows up. Both bandnotches were studied simultaneously in order to study the effect of the band notches on each other in additionto their effect on the entire bandwidth.To study the band notch characteristics of the antenna, it is convenientto illustrate the voltage standing wave ratio (VSWR) instead of the reflection coefficient. Figure 5a shows theeffect of varying the length of the quarter wavelength parasitic element. Increasing the length of the parasiticelements shifts the WLAN band notch to the right following the well-known equation given below [20]:
L =λ
4√
ϵr+12
, (1)
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(a)
2 4 6 8 10 12 -50
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Frequency (GHz)
|S11
|(d
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L1=0mm L1=2mm
L1=4mm L1=6mm
Frequency (GHz)
(b)
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50
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20
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0
10
|S11
|(d
B)
L2=1mm L2=2mm
L2=4mm L2=6mm
Frequency (GHz)
Figure 3. Effect of the matching stub on the antenna reflection coefficient with both groups of PIN diodes are OFF,L1 = 4 mm, L2 = 4 mm, (a) S = 2 mm and different values of H (b) H = 6 mm and different values of S.
(a)
2 4 6 8 10 12 -4
0
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30
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20
-10
0
10
Frequency (GHz)
|S11
|)
Bd (
-
H=2mm H=4mm
H=6mm
H=8mm
(b)
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1
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Frequency (GHz)
|S11
|(d
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4
S=0mm S=1mm S=2mm S=3mm
Figure 4. Effect of the matching stub on the antenna reflection coefficient with both groups of PIN diodes are OFF,L1 = 4 mm, L2 = 4 mm, (a) S = 2 mm and different values of H (b) H = 6 mm and different values of S.
where L denotes the overall length of the parasitic element, λ represents the wavelength in the freespacecorresponding to the resonant frequency of the notch, and ϵr is the dielectric constant of the substrate. Ford1 value equal to 2.5 mm, the band notch resonant frequency is 5.4 GHz along the frequency range (5.1–5.8GHz) which perfectly covers the frequency spectrum of the WLAN applications (5.15–5.725 GHz). In this case,the overall length of each quarter wavelength parasitic element is 10.5 mm. In addition, the VSWR value ofthe band notch at its resonant frequency is found to be 13.7, which is high enough to perfectly eliminate theinterference with the WLAN radiation. The presence of this band notch has no effect on the overall bandwidth(frequency range with VSWR ≤ 2) of the antenna except at the notched frequency. However, these parasiticelements have minor effect on the matching of the other band notch (see Figure 4a). This is because the quarterwavelength parasitic element is connected to the ground plane and this leads to modify the electromagneticcoupling between the half wavelength parasitic elements and the ground plane.
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(a)
2 3 4 5 6 7 8 9 10 11 12 2 1
5
10
15
20
03
52
Frequency (GHz)
RW
SV
d1=1mm
d1=1.5 mm
d1=2mm
d1=2.5 mm
(b)
2 3 4 5 6 7 8 9 10 11 12 2 1
5
10
15
20
03
52
Frequency (GHz)
RW
SV
d2=1mm
d2=1.5 mm
d2=2mm
d2=2.5 mm
Figure 5. Effect of (a) the quarter wavelength parasitic elements (d2 = 2.5 mm and different values of d1) and (b) thehalf wavelength parasitic elements (d1 = 2.5 mm and different values of d2) on the VSWR of the proposed antenna withboth groups of PIN diodes are ON.
The effect of the half wavelength parasitic elements is demonstrated in Figure 5b. Varying the value of d2results in changing the position of the band notch since it changes the overall length of the parasitic elements.When d2 = 2.5 mm, the overall length of each half wavelength parasitic element becomes 19 mm. At this value,the overall length of the parasitic element follows the familiar equation given below [20]:
L =λ
2√
ϵr+12
. (2)
The resulting band notch is centered at 7.3 GHz covering the frequency range (6.9–7.8 GHz) with VSWR valueapproaching to 20 which entirely rejects the downlink of the military X-band satellite communications (7.25–7.745 GHz). The trivial effect of the presence of this band notch on the entire antenna bandwidth and theWLAN band notch can easily be sensed from Figure 5b, and this is a very important outcome for the proposedantenna.
The reason behind the generation of each band notch can be exposed by demonstrating the currentdistribution at the center frequency of each band notch. Figure 6 exhibits the current distribution of theproposed antenna at 5.4 GHz when both PIN diode groups are ON. The current is concentrated at the quarterwavelength parasitic elements and moves in two opposite directions. This results in two opposite radiationsthat cancel each other. Figures 7a and 7b illustrate the current distribution at the front and back sides of theproposed antenna, respectively, at 7.3 GHz when both PIN diode groups are ON. The current is concentratedat the circumference of the matching stub in the front side of the antenna and at the half wavelength parasiticelements in the back side of the antenna. This current distribution elucidates that the half wavelength parasiticelements break the coupling between the feed line and the radiating wide slot by making stronger coupling withthe tuning stub.
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-50.5
A/m
-41.3
-32.1
-23
0
-13.8
13.8
23
32.1
41.3
50.5
Figure 6. Current distribution of the proposed antenna at 5.4 GHz when PIN diode Group 1 is ON.
(a)
-50.5
A/ m
-41.3
-32.1
-23
0
-13.8
13.8
23
32.1
41.3
50.5
(b)
-50.5
A/ m
-41.3
-32.1
-23
0
-13.8
13.8
23
32.1
41.3
50.5
Figure 7. Current distribution of the proposed antenna at 7.3 GHz when PIN diode Group 2 is ON (a) front view and(b) back view.
4. Measured resultsFigures 8a and 8b show the front and back views, respectively, of the prototype of the proposed wide-slotUWB antenna. The measurements were performed at the Department of Electrical Engineering at Universityof Basrah using AMITEC network analyzer package. The biasing circuit of each group of PIN diodes is shownin Figure 9; the circuit includes a switch to turn the diodes ON and OFF as well as a chocking inductance toprevent the RF current from passing through the DC voltage supply. The PIN diodes that are used in thisdesign are SMP1320-079LF whose reverse biasing capacitor is (Cr = 0.3 pF), and forward resistance value is(Rf = 0.9 Ω at 10 mA). Each diode is switched on by passing 10 mA through it, and it can be switched off byseparating the DC voltage source by the external switch. The diodes of each pair are switched on by sharingthe 20 mA equally since the two diodes are identical. However, the other pair has another similar biasingcircuit that controls its operation separately. The deviation between the simulated and measured results maybe attributed to the imperfect soldering of the SMA connector, misalignment of the PIN diodes, biasing linesunintended coupling, or the fabrication tolerations.
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(b)
PIN
Group 1
PIN
Group 2
PI
(a)
Figure 8. The prototype of the proposed antenna with the biasing lines of the PIN diodes (a) front view and (b) backview.
Chocking
inductor 500Ω 20m
A
10
Figure 9. The biasing circuit for each PIN diode group.
The VSWR of the proposed antenna at different PIN diode states is exhibited in Figure 10. When bothgroups of PIN diodes are OFF (see Figure 10a), the antenna bandwidth (VSWR ≤ 2) perfectly covers theentire frequency range of the UWB applications. When Group 1 is turned on (see Figure 10b), the WLAN bandnotch shows up, while the X-band rejection appears when Group 2 is turned ON (see Figure 10c). Intuitively,turning both groups ON produces the dual-band notches as shown in Figure 10d. The measured VSWR valueat the resonant frequency of each band notch approaches 12 for the WLAN rejection and to 17 for the X-bandrejection. Figure 11 illustrates the simulated and measured gain of the proposed antenna for different statesof the PIN diode groups. When both diode groups are OFF, the gain value is stabilized around 4.5 dBi asrevealed in Figure 11a. Turning both diode groups on does not affect any of the frequencies outside the notched
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bands, but at the notched band the gain value is reduced to about −2 dBi for the WLAN notched band and to−6 dBi for the X-band notched band (see Figure 11b). The antenna power pattern of the proposed wide-slotantenna for three different resonant frequencies are shown in Figures 12a–12c. The E-plane (yz-plane) patternfor the three frequencies is bidirectional, while the H-plane (xz-plane) pattern is almost omnidirectional. As aresult, the antenna is very compatible with the portable underlay CR devices which require antennas that areindependent of position.
(a)
2 3 4 5 6 7 8 9 10 11 12 2 1
5
10
15
20
03
52
Frequency (GHz)
RW
SV
Measured MeasuredSimulated
(b)
2 3 4 5 6 7 8 9 10 11 12
1
5
10
15
20
0
3
52
Frequency (GHz)
RW
SV
Measured MeasuredSimulated
(c)
2 3 4 5 6 7 8 9 10 11 12
1
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3 5
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Measured MeasuredSimulated
(d)
2 3 4 5 6 7 8 9 10 11 12
1
5
10
15
20
0
3 5
2
Frequency (GHz)
RW
SV
Measured MeasuredSimulated
Figure 10. Simulated and measured VSWR of the proposed antenna for different states of PIN diode (a) Group 1 andGroup 2 are OFF, (b) Group 1 is ON and Group 2 is OFF, (c) Group 1 is OFF and Group 2 is ON and (d) Group 1and Group 2 are ON.
The time domain characteristics of the antenna can be obtained by calculating the pulse fidelity ofthe transmitted pulse. As discussed in detail in [21], the pulse fidelity represents the correlation coefficientbetween the transmitted pulse (fourth order Rayleigh pulse) and the signal receive by another copy of theproposed antenna. The pulse fidelity is found to be 0.796 without the band notch and 0.6825 with the dual-band notch. Both values of pulse fidelity are acceptable for UWB applications since they are larger than 0.5[21]. The reduction of the pulse fidelity with the presence of the band notch is attributed to the distortion
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(a)
2 3
4
5
6 7
8
9
10
11
12
2
-6
-4
-2
0
2
4
6
8
Frequency (GHz)
)iB
d(ni
aG
Measured Measured
Simulated
(b)
2 3
4
5
6 7
8
9
10
11
12
-6
-4
-2
0
2
4
6
8
Frequency (GHz)
)iB
d(ni
aG
Measured Measured
Simulated
Figure 11. Simulated and measured realized gain of the proposed antenna for different states of PIN diode (a) Group1 and Group 2 are OFF and (b) Group 1 and Group 2 are ON.
that the antenna accumulates at the notched frequency bands to eliminate their radiation. Table summarizesthe proposed antenna specifications compared to some important antenna designs with dual-band notch. Thesuperiority of the proposed antenna over other designs is concluded by its small size, sharp band notch, andreduced gain values at the notched bands.
Table. Comparison between the proposed UWB reconfigurable antenna with a previously published significant work.
Antenna Dimensions mm3 WLAN notch X-band notch WLAN gain X-band gain FrequencyS11(dB) S11(dB) (dBi) (dBi) band(GHz)
5. ConclusionTwo pairs of parasitic elements with two pairs of PIN diodes are attached to a wide-slot UWB antenna to designUWB antenna characteristics with dual reconfigurable band notches for underlay CR applications. In additionto the sharpness of the band notch, the frequency span of the band notch is wide enough to eliminate theentire 5-GHz WLAN and the downlink of the military X-band satellite communications. The results show theantenna’s perfect coverage for the frequency band of the UWB applications in the absence of the band notch.The VSWR of the WLAN band notch reaches a value greater than 12, and for the X-band military band notch,the VSWR surpasses 17. The gain of the proposed antenna is around 4.5 dBi outside the band notches, whereasits value reduced to −2 dBi and −6 dBi at the WLAN and X-band frequencies, respectively. The antenna alsohas almost stable omnidirectional radiation pattern over the entire antenna bandwidth suitable for portabledevices.
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(a)
z0
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red
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MeeasuredSimulated
(b)
90
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330
90 60
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1801800
300 270
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H-Plane E-Plane
Figure 12. Simulated and measured normalized power pattern of the proposed antenna at (a) 3.4 GHz, (b) 6.8 GHz,and (c) 10 GHz.
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