1732 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 6, JUNE 2007
Reduction of Mutual Coupling BetweenClosely-Packed Antenna Elements
Chi-Yuk Chiu, Member, IEEE, Chi-Ho Cheng, Ross D. Murch, Senior Member, IEEE, andCorbett R. Rowell, Member, IEEE
Abstract—A simple ground plane structure that can reducemutual coupling between closely-packed antenna elements isproposed and studied. The structure consists of a slitted pattern,without via’s, etched onto a single ground plane and it is thereforelow cost and straightforward to fabricate. It is found that isola-tions of more than 20 dB can be achieved between two parallelindividual planar inverted-F antennas (PIFAs) sharing a commonground plane, with inter-antenna spacing (center to center) of0.116 o and ground plane size 0.331 2
o. At 2.31 GHz it is demon-
strated that this translates into an edge to edge separation betweenantennas of just 10 mm. Similarly the structure can be applied toreduce mutual coupling between three or four radiating elements.In addition the mutual coupling between half wavelength patchesand monopoles can also be reduced with the aid of the proposedground plane structure. Results of parametric studies are alsogiven in this paper. Both simulation and measurement resultsare used to confirm the suppression of mutual coupling betweenclosely-packed antenna elements with our slitted ground plane.
THE mutual coupling or isolation between closely packedantenna elements is important in a number of applications.
These include systems relying on array antennas and more re-cently multiple input multiple output (MIMO) wireless com-munication systems which rely on multiple antennas to offerincreases in capacity without the need for additional power orspectrum, compared to conventional systems [1]. Achieving lowmutual coupling between closely-packed antenna elements isdifficult to achieve and has been well studied. For example in[2] it is stated that no matter how two PIFAs [3], [4] are orientedeither collinear, orthogonal or parallel above a single groundplane with air substrate, the inter-element spacing should be atleast greater than 0.5 where is the free space wavelengthat the center frequency, for an isolation of at least 20 dB. Toovercome these limitations researchers have found that mush-room-like EBG structures are able to suppress surface wavepropagation [5]–[7], and thus reduce mutual coupling betweenradiating elements. They involve an intricate fabrication process
Manuscript received September 12, 2006; revised February 5, 2007.C. Y. Chiu, C.-H. Cheng, and R. D. Murch are with the Department of Elec-
tronic and Computer Engineering, The Hong Kong University of Science andTechnology, Clear Water Bay, Kowloon, Hong Kong (e-mail: [email protected]).
C. R. Rowell is with the Applied Science and Technology Research Institute,Hong Kong.
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2007.898618
with cells shorted to the ground through vias. Additionally re-searchers have found that the defected ground structure (DGS)is also able to provide a bandstop effect due to the combina-tion of inductance and capacitance [8], [9]. It has been appliedto antenna designs to suppress harmonics, cross polarization ofa patch antenna, and to increase the isolation of a dual-polar-ized patch antenna [10]–[12]. An altogether different approachfor closely packing antennas has been proposed in [13] whichconsists of a compact integrated diversity antenna with two feedports, but it does not provide a general solution for reducing mu-tual coupling between common antenna elements.
This paper presents a new structure to improve the isolation ofa pair of closely-packed PIFAs, patch and monopole antennas ona common ground plane, with edge to edge spacing smaller than0.09 . The proposed technique is to make use of the groundplane itself to provide a filter effect. A pattern etched onto theground plane between antenna elements effectively suppressesmutual coupling. Significant improvement in isolation betweenantenna elements can be obtained when compared to the casewith a conventional ground plane. The method is different fromthe EBG structure, because only one layer of ground plane isneeded and it does not require an intricate fabrication process.The structure is also effective for monopole antennas. It is foundthat an isolation of 20 dB can be obtained between a pair ofmonopole antennas with an antenna to antenna separation of0.084 . In addition, the proposed idea can be applied to reducemutual coupling to between more than two radiating elements.
In Section II we describe the proposed ground plane structureand demonstrate its effectiveness when configured with twoclosely packed PIFAs. In Section III we further investigate theproposed ground structure with parametric studies, equivalent
structures as well as the surface current distribution. InSection IV we then propose various configurations of closelypacked antenna structures including up to four PIFAs, halfwavelength patch and also monopole antennas. Results in-cluding -parameters, radiation patterns, gain and efficiency ofthe antennas are given. Finally in Section V, a conclusion ofthe proposed ground plane structure is presented.
II. PROPOSED GROUND PLANE STRUCTURE GEOMETRY
The proposed ground plane structure can be seen in Fig. 1.In general, it consists of N pairs of slits of length and width
and these are etched into the middle of a ground plane asshown. In addition the N-1 pairs of metal strips between the slitshave width and are connected to the central conductor bar ofwidth .
For the purpose of analyzing and understanding the effec-tiveness of the proposed ground plane structure it is useful
CHIU et al.: REDUCTION OF MUTUAL COUPLING BETWEEN CLOSELY-PACKED ANTENNA ELEMENTS 1733
Fig. 1. Geometry of two closely-packed PIFAs with proposed ground planestructure.
Fig. 2. Simulated and measured S-parameters of two closely-packed PIFAswith proposed ground plane.
to provide a specific test example of how it can be used withclosely packed antennas. In Fig. 1, we show the structure con-figured between two PIFAs when five pairs of slits ( ,
, and ) are etched in the middleof the ground plane. The ground plane and the radiating patchesare made from copper sheet with thickness of 0.2 mm. Eachpatch is supported by two shorting posts and a coaxial probefeed in the air. To aid the reduction of the inter-element spacing,the PIFAs are excited from different sides. The antennas operateat 2.305 GHz with dimensions of ,
, ,, . The slits
etched in the middle of the ground plane are basically used toreduce mutual coupling between radiating elements.
III. GROUND PLANE STRUCTURE STUDIES
To investigate and better understand the operation of the pro-posed ground plane structure described in Section II, it is simu-lated by the commercial software package HFSS [14], with theperformance additionally verified by experiments. In Section IVresults from a variety of other configurations for the antenna ge-ometry are then considered.
A. Two PIFAs Configuration
Fig. 2 shows the results of simulated and measured -param-eters against frequency for the two PIFAs structure in Fig 1.
Fig. 3. Simulated and measured S-parameters of two closely-packed PIFAs:(a) with conventional ground plane, (b) separated ground plane.
The measured impedance bandwidth ( less than 10 dB) is3.46% from 2.27 to 2.35 GHz, with mutual coupling across theband better than 20 dB. For comparison, two more configura-tions have been investigated in this section. They are: 1) PIFAslocated on a solid conventional ground plane, and 2) PIFAs lo-cated on two separated ground planes (i.e. all the ribs in themiddle portion of the ground plane depicted in Fig. 1 are re-moved). The simulated and measured -parameters results aregiven in Fig. 3. It can be observed that the isolation of our newproposed ground geometry provides a significant improvementof isolation of 15 dB and 7.5 dB over the cases with a solidconventional ground plane and two separated ground planes, re-spectively. The degenerated resonant mode of the antennas canbe eliminated by increasing the inter-element spacing. However,for the conventional ground plane design, the will still re-main poor at 7 dB across the operating frequency band evenwhen the spacing is doubled. The far-field pattern measurementsare provided in Fig. 4(a) and (b), where the peak gain and effi-ciency are 3.66 dBi and 88%, respectively and were measuredby StarLab [15]. In these measurements one of the PIFAs inthe prototype is terminated with 50 ohms load. It should alsobe noted that the antenna structure we constructed uses a smallpiece of material (acrylic) at the back side of the ground planeto steady the structure during measurement. The acrylic blockwe used is Polymethyl Methacrylate with relative permittivityof 2.8 and loss tangent of 0.03. Simulations of efficiency reveal
1734 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 6, JUNE 2007
Fig. 4. Measured radiation patterns of PIFAs with proposed ground plane struc-ture at 2.31 GHz: (a) x� z plane, (b) y� z plane. Measured radiation patternsof PIFAs with conventional ground plane at 2.24 GHz: (c) x�z plane, (d) y�z
plane.
that the block is the main cause for the efficiency loss of thestructure.
When compared to radiation patterns in which a conventionalground plane is used [see Fig. 4(c) and (d)] the gain and ef-ficiency are reduced to 0.98 dBi and 49%, respectively. Thisis primarily caused by the higher mutual coupling in the con-ventional ground plane design. It should also be noted that thepatterns themselves do not differ significantly in the two con-figurations. The radiation pattern shape in the back and forwardplanes remains similar in both cases and this is most likely dueto the small ground plane in both designs.
B. Parametric Studies
From the results of the two PIFAs case, it can be seen thatthe slitted ground plane structure has a very positive impact onisolation. To understand the design tradeoffs of the proposedstructure, we have also performed parametric studies of theslitted ground plane structure. Table I is a summary of antennaperformance with different inter-element spacing and numberof slit pairs used on the ground plane. The study is based onthe geometry given in Fig. 1 with PIFAs in parallel position.The ground plane size is kept at 43 mm 43 mm and sim-ulation is performed throughout the study. The table detailsthe inter-element spacing, number of slit pairs used, centerfrequency of the PIFAs, operating impedance bandwidth andmaximum mutual coupling within operating frequency band.It can be observed that for center to center spacings of greaterthan 0.12 wavelengths isolations of better than 15 dB can
TABLE ISUMMARY OF MAXIMUM MUTUAL COUPLING BETWEEN TWO PARALLEL
PIFAs WITH DIFFERENT INTER-ELEMENT SPACING AND SLIT PAIRS
be achieved. Additionally the effects on resonant frequenciesare slight for different numbers of slits. For separations of lessthan 0.12 wavelengths it can be observed that both bandwidthand isolation deteriorate suggesting that 0.12 wavelengths isapproximately the useful limit for our configuration. For thetwo PIFAs case with center to center separation of 17 mm,the optimum number of pairs of slits is five. When one moreslit pair is etched on the ground plane, the isolation drops byabout 6 dB. A similar result is found for another case withcenter to center separation of 19 mm. The isolation goes upto a maximum value and then drops down as we increase thenumber of slit pairs. Therefore, according to the table, it isnoted that even though more slit pairs are etched on the groundplane, it may not have the best performance for suppressingmutual coupling.
C. Equivalent Structures
Referring to Fig. 1, the proposed ground plane structure con-sists of slits interleaved with strips and these strips could bethought of as capacitors. At the same time, some inductanceis introduced along the central small connecting strip. There-fore, we could think of the structure as equivalent to a band-stop filter based on a parallel resonator. To demonstrate thebandstop effect of the proposed pattern, a structure consisting offive pairs of slits with dimensions the same as that found in theground plane shown in Fig. 1 is printed on an FR-4 substrate,and connected to two microstrip lines at both ends. For a betterapproximation, the metal under the slitted pattern is scratchedout. A bandstop region is observed from 2.07 GHz to 2.59 GHzwith isolation better than 10 dB as illustrated in Fig. 5(a). Italso shows that there is a good agreement with those small areasreplaced by lumped inductors with values of 0.1 nH each as de-picted in Fig. 5(b). If larger values of lumped inductors are se-lected, for example 5 nH each, the bandstop region shifts to alower frequency band. In fact, the capacitance can be also in-creased by reducing the slit width or increasing the slit length,
CHIU et al.: REDUCTION OF MUTUAL COUPLING BETWEEN CLOSELY-PACKED ANTENNA ELEMENTS 1735
Fig. 5. Frequency responses of slitted pattern connected with two microstriplines, where strips are connected by (a) small metal areas, (b) lumped inductors.
so that the bandstop region can be further shifted to lower fre-quency band.
D. Surface Current Distribution
Given the parametric studies and equivalent structures wehave described in the previous two sections it is also good tobetter understand its mechanism by investigating the surfacecurrent density. We have performed this by plotting the currentdistribution over the surface of the ground plane. As shown inFig. 6 (only the current density on the ground plane is shown),a large portion of surface current is being trapped by the firstground plane slit next to the radiating patch. Less current ispropagating across the slitted pattern. This demonstrates thatcurrent flows from one side of the ground plane to the otherside are substantially reduced and this effectively helps to re-duce mutual coupling between the two PIFAs. Such an inter-pretation also fits with the bandstop filtering characteristics de-scribed in Section III-C.
IV. CLOSELY PACKED ANTENNA CONFIGURATIONS
To illustrate the versatility of our proposed ground planestructure we also show how it can be used in a variety ofother antenna configurations. This includes monopoles, halfwavelength patches as well as more than two PIFAs.
A. Two Monopole Antennas Configuration
It has already been shown that the proposed ground planestructure is able to suppress mutual coupling between two
Fig. 6. Simulated surface current distribution at 2.35 GHz (top view).
Fig. 7. Geometry of two closely-packed monopole antennas. (Units: mm).
closely-packed planar radiating elements. However, we havealso found that the isolation between two straight monopoleantennas separated by 0.093 (center to center) of betterthan 20 dB can be achieved when our slitted ground plane isused. Fig. 7 shows two monopole antennas constructed fromcopper wires with lengths and diameters of 32.5 mm and 1 mm,respectively. They are both standing on top of a slitted groundplane with slit size of 11 mm 1 mm each. The antennas areresonating at 2.53 GHz with isolation of 21.8 dB as shownin Fig. 8. Compared to the -parameters of two monopoleantennas with the same separation (11 mm) but sharing a solidconventional ground plane (40 mm 25 mm), 15.9 dB isola-tion improvement is achieved. If the two monopole antennasare placed on top of the proposed ground plane and bent awayfrom each other at right-angles into an L-shape, we also foundthat good isolation can be still maintained within the operatingfrequency band, as long as the horizontal part of the monopolesare not too close to the ground plane. This shows that our
1736 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 6, JUNE 2007
Fig. 8. Comparison of measured S-parameters between closely-packedmonopole antennas with conventional and slitted ground planes.
Fig. 9. Geometry of two closely-packed rectangular patch antennas. (Units:mm).
proposed technique is also effective in suppressing mutualcoupling between two closely-packed non-planar radiating ele-ments. It should also be mentioned that alternative techniquesfor reducing coupling between monopoles have also beenproposed. One particular example is reported in [16], where asloping monopole configuration is also found to reduce mutualcoupling.
B. Two Rectangular Patch Antennas Configuration
Fig. 9 shows two patches printed on a thickness of 1.6 mm di-electric substrate with relative permittivity of 4.6. The patchesare designed for resonating at 0.97 GHz with center to centerinter-element spacing of 0.265 , which is equivalent to thespacing between the two individual coaxial probe feeds. Here,same side excitation for the patches will be used as the spacingis relatively large when compared to the two parallel PIFAs case.Since no shorting post or wall is connected to the radiating ele-ments, the longest sides of the patches in our model are approx-imately half wavelength. Two pairs of slits with size of 56.5 mm
4 mm are etched on the ground plane as given in the geometry.Experiments show that there is a large improvement of 10.6 dBmutual coupling at the resonant frequency between patches withslitted and conventional ground plane structures as depicted inFig. 10. It is confirmed that the slitted pattern depends on thefrequency and antenna type. Different sizes of slits and stripsare required for reducing mutual coupling of different resonantfrequency and type of the antennas.
Fig. 10. Comparison of measured S-parameters between closely-packed rect-angular patch antennas with conventional and slitted ground planes.
Fig. 11. Measured S-parameters of two PIFAs in orthogonal position: (a) withslitted ground plane, (b) conventional ground plane.
C. PIFAs Configurations
Three & Four PIFA Elements: It is also found that by ap-plying the proposed slitted ground plane structure, the isolationbetween patch antennas with three and four radiating elementscan be improved as well. However, as the number of elements isincreased, more complicated mutual coupling will be presented.Therefore, the minimum inter-element spacing becomes largercompared to the case with two radiating elements only. Oneexample is that when three PIFAs (same antenna dimensionsas given in Fig. 1) are oriented in parallel with alternate sidefeeds, ground plane size of 103 mm 43 mm, and inter-ele-ment spacing of 40 mm, the isolation can be improved by 12 dBacross the operating frequency band if 5 pairs of slits are used
CHIU et al.: REDUCTION OF MUTUAL COUPLING BETWEEN CLOSELY-PACKED ANTENNA ELEMENTS 1737
on the ground plane in between any two radiating elements. Ifone more radiating element is packed with ground plane size of143 mm 43 mm, such that four PIFAs are lined up in x-di-rection (referring to Fig. 1), there is also 12 dB mutual couplingreduction between cases with and without slitted ground planestructure.
PIFAs Orientation: With antenna 1 as shown in Fig. 1rotated anti-clockwise by 90 , such that two PIFAs are orientedorthogonally, a good mutual coupling suppression can also beobtained by applying our proposed ground plane structure. Theedge to edge spacing and ground plane size are now 10 mmand 64 mm 43 mm, respectively. And all other parametersremain unchanged. According to the measurement result shownin Fig. 11(a), 20 dB isolation can be achieved across the band.Since and are identical, therefore, only would beshown in the figure. However, as the two PIFAs are in differentpositions with respect to the stripped pattern, the resonantfrequencies are slightly different to each other. To tackle thisminor problem, a slight adjustment on the radiating patch maybe required.
V. CONCLUSION
This paper presents and demonstrates a new structure for re-ducing mutual coupling between closely-packed antenna ele-ments. The proposed structure itself is simple and does not re-quire any special design for the radiating elements be it a patch,PIFA or monopole. Together with the combination of capaci-tance and inductance between the slits, a good suppression ofmutual coupling between the antennas can be achieved. Thestructure can be thought of as a bandstop filter between theantennas and this effectively reduces current flow between theantennas. The new ground plane structure is also desirable forimproving isolation between two PIFAs in parallel or orthog-onal orientation. By properly designing the slits on the groundplane, the proposed technique is applicable to patch antennasbuilt on different substrates with or without shorting posts, ormonopoles. The technique may be useful in a number of tech-nologies including MIMO antennas and arrays but this will needfurther investigation.
ACKNOWLEDGMENT
The authors sincerely appreciate the help of the Compact An-tenna Team, at the Applied Science and Technology ResearchInstitute, Hong Kong, for providing near-field measurements.
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Chi-Yuk Chiu (S’01–M’05) received both theB.Eng. and M.Eng. degrees, and the Ph.D. degreein electronic engineering from the Department ofElectronic Engineering, City University of HongKong, in 2001 and 2005, respectively.
He is currently a Research Associate at the Depart-ment of Electronic and Computer Engineering, HongKong University of Science and Technology. His cur-rent research interests include the design and analysisof small antennas, MIMO antennas and MIMO mea-surements.
Dr. Chiu serves as a Reviewer of the IEEE TRANSACTIONS ON WIRELESS
COMMUNICATIONS.
Chi-Ho Cheng received the B.Eng. degree from TheHong Kong University of Science and Technology(HKUST), in 2005, where he is currently pursuingthe M.Phil. degree.
He was a Research Assistant developing RFIDtag and reader antenna and MIMO systems during2006–2007 in the Department of Electronic andComputer Engineering, HKUST. He is currently aResearch Assistant working on RFID benchmarkingproject.
1738 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 6, JUNE 2007
Ross D. Murch (M’84–SM’98) is a Professor ofelectronic and computer engineering at the HongKong University of Science and Technology. He isalso the Founding Director of the Center for WirelessInformation Technology at Hong Kong University ofScience and Technology which was begun in August1997. He is also the Program Director for the M.Sc.in Telecommunications at Hong Kong University ofScience and Technology. From August-December1998 he was on sabbatical leave at Allgon MobileCommunications (which manufactured 1 million
antennas per week in 1998), Sweden and AT&T Research Labs, NJ. His currentresearch interests include MIMO antenna design, MIMO and cooperativesystems, WLAN, B3G and ultrawideband (UWB) systems for wireless com-munications. He has several U.S. patents related to wireless communication,over 150 published papers and acts as a consultant for industry and government.
Prof. Murch is an area Editor for the IEEE TRANSACTIONS ON WIRELESS
COMMUNICATIONS, Technical Program Chair for the IEEE Wireless Communi-cation and Networking Conference in 2007, and was the Chair of the AdvancedWireless Communications Systems Symposium at ICC 2002.
Corbett R. Rowell (M’96) received the B.A. degreein physics from the University of California, SantaCruz, in 1994, and the M.Phil. in electrical andelectronic engineering from Hong Kong Universityof Science and Technology, in 1996.
From 1997 to 1999, he worked as an RF Engi-neer at Allgon Mobile Communications, in Sweden.In 1998, he started his own antenna design company,Integra Antennas Ltd., and sold part of it in 2003 toMolex Inc. From 2001 to 2003, he worked on WallStreet at JP Morgan as a Technical Expert in Venture
Capital. From 2003 to 2005, he was a Senior Antenna Engineer at Molex Inc.,in Hong Kong. Currently, he is the R&D Director of RF and Antennas at theApplied Science and Technology Research Institute in Hong Kong. He has pub-lished over 10 papers and holds over 20 patents (including patents pending).His research interests are miniature antennas, antenna arrays, active antennas,beam-forming, and isolation.
