01. Dual-Band Dual-Polarized Compact Bowtie Antenna Array for Anti-Interference MIMO WLAN - Zheng, Zhang, Li - 06646242

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014 237

Dual-Band Dual-Polarized Compact Bowtie AntennaArray for Anti-Interference MIMO WLANWen Chao Zheng, Long Zhang, Qing Xia Li, Member, IEEE, and Yi Leng, Member, IEEE

AbstractSmart antennas have received increasing interestfor mitigating interference in the multiple-inputmultiple-output(MIMO) wireless local area network (WLAN). In this paper, adual-band dual-polarized compact bowtie dipole antenna array isproposed to support anti-interferenceMIMOWLAN applications.In the antenna array, there are 12 antennas, six for horizontalpolarization and six for vertical polarization. In order to achievedual linear polarizations and beam switching, six horizontalantennas are placed in a sequential, rotating arrangement on ahorizontal substrate panel with an equal inclination angle of 60to form a symmetrical structure, while the other six antennasfor vertical polarization are inserted through slots made on thehorizontal substrate panel. Furthermore, six pairs of meanderedslits are introduced to reduce the mutual coupling between hori-zontal antennas in the lower band. A prototype of the array with adimension of 150 150 60 mm is manufactured and exhibitsthe characteristics of high isolation, good front-to-back ratio, andaverage gains of 4.5 and 5 dBi over the 2.4- and 5-GHz band,respectively. The MIMO performance of the array is analyzedand evaluated by mutual coupling, the total active reflectioncoefficient (TARC) and the envelope correlation coefficient. Theanti-interference capability of the array is also investigated by theexperiment.

Index TermsAntenna array, anti-interference, bowtie, com-pact, dual-band, dual-polarization, MIMO, WLAN.

I. INTRODUCTION

H IGH data rate and anti-interference capability are thenecessary characteristics for the wireless local areanetworks (WLANs). Multiple-inputmultiple-output (MIMO)antenna systems have attracted considerable interests as aneffective way of improving the data rate and increasing thechannel capacity in WLANs [1]. However, MIMO WLANsystems suffer severe interference problems when more andmore wireless access points (APs) have been deployed orthe number of WLAN users are abruptly increasing [2], [3].Smart antenna technology is an effective way to overcome thisflaw [4], [5]. Switched beam and adaptive beamforming arrays

Manuscript received February 04, 2013; revised September 03, 2013; ac-cepted October 11, 2013. Date of publication October 24, 2013; date of currentversion December 31, 2013. This work was supported in part by the NationalNatural Science Foundation of China under Grants 41176156, 41275032, and61201123.W. C. Zheng, L. Zhang, and Q. X. Li are with the Science and Technology

on Multi-Spectral Information Processing Laboratory, Huazhong University ofScience and Technology, Wuhan 430074, China (e-mail: [email protected]; [email protected]; [email protected]).Y. Leng is with the Department of Information Counter, Air Force Early

Warning Academy, Wuhan 430019, China (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TAP.2013.2287287

are two main smart antenna technologies. Switched beamarrays with directional antennas [6], [7] have the advantage ofsimplicity since several fixed beams could be chosen to reducethe interference by controlling the state of a number of RFswitches. Compared to the adaptive beamforming arrays, thesimplicity of the switched beam arrays makes it suitable forlow-cost, low-power applications in anti-interference MIMOWLANs.MIMOWLAN antenna arrays with directional antennas have

been developed [8][11]. In [8] and [9], two different kinds ofhigh-gain, dual-loop antennas were applied to three-antennasystems for MIMO AP applications. A printed YagiUdaantenna with integrated balun was reported, and an MIMOarray was obtained in a triangular configuration at 5.2-GHzband [10]. Moreover, a two-port dual-band dual-polarizationarray [11] was proposed for MIMO WLAN. However, theseantenna arrays do not support beam switching for anti-interfer-ence applications.This paper presents a dual-band dual-polarized compact

bowtie dipole antenna array for MIMOWLAN, which supportsbeam switching. In the array, there are 12 antennas, six forhorizontal polarization and six for vertical polarization. Sixantennas for horizontal polarization are placed in a sequential,rotating arrangement on a horizontal substrate panel with anequal inclination angle of 60 to form a symmetrical struc-ture, while the other six antennas for vertical polarization areinserted through slots made on the horizontal substrate panel.Furthermore, six pairs of meandered slits are introduced toreduce the mutual coupling between horizontal antennas in thelower band. Each of the 12 antennas comprises two bowties,a director, a microstrip line to feed the antenna, a widebandtransition from the microstrip line to a parallel stripline (PSL),and a ground plane.Compared to the existing MIMO WLAN antenna arrays, the

proposed array has the advantage of compact structure, duallinear polarizations, high isolation, and anti-interference capa-bility. The compact structure of the arraymainly results from thefeeding structure and the dual linear polarizations. In the array,the high isolation is obtained by two kinds of decoupling strate-gies, which are meandered silts on the ground and dual-polar-ization arrangement. In addition, the anti-interference character-istic of the array is mainly due to the radiation pattern featuresof the 12 sector antennas.A prototype of the array with a dimension of

150 150 60 mm is manufactured to support threedata streams system with beam switching over the 2.4-GHzband and the 5-GHz band. In Section II, the geometry anddesign consideration of the array are described. In Section III,advantages of the array are demonstrated by the measured and

0018-926X 2013 IEEE

238 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014

Fig. 1. Coupling between the vertical antennas.

computed parameters, including -parameters, total activereflection coefficient (TARC), envelop correlation coefficients,radiation pattern, and signal-to-interference ratio (SIR). Finally,our conclusion is presented in Section IV.

II. DUAL-BAND DUAL-POLARIZED COMPACT BOWTIEANTENNA ARRAY

The proposed dual-band dual-polarized antenna array con-sists of 12 compact antennas. The dual-band compact bowtieantenna is first introduced. Then, a whole dual-polarized an-tenna array, which consists of 12 of the compact antennas, ispresented.

A. Compact Directional AntennaThe single antenna is a double-bowtie dipole structure. Fig. 1

shows the geometry of the proposed antenna. The antenna con-sists of two bowties, a director, a microstrip line to feed theantenna, a transition from the microstrip line to a PSL, and aground plane. Each bowtie has two printed arms, one on the topand the other on the bottom of the substrate. The radii (R2 andR3) and the flare angles of the arms control the resonance fre-quencies. The benefit of the bowtie shape is to reduce the sizeof the antenna compared to a normal dipole antenna.The single antenna has a simple feeding structure. In the

feeding part of the antenna, the top and bottom arms of bowtiesare connected to the PSL, while the PSL is linked to themicrostrip line through a wideband transition. This transitiontapers from the ground plane to the width of the PSL with amanner of quarter-circles, which has a simple geometry. Thebest matching can be achieved just by tuning the parameter R1.With this tapered wideband transition, the proposed antennahas wider bandwidth and smaller size than other designs withbalun in [10], [12] because the balun in these designs is alwaysbased on a half-wavelength delay line, which is designed at thecenter frequency.Moreover, the directional radiation pattern of the single an-

tenna is owing to its double quasi-Yagi structures. The smallerbowtie, which has a quarter-wavelength (at the center frequencyof the higher band) radius, is considered to be a dipole workingat higher band. Since the bigger bowtie can act as a reflector, aquasi-Yagi antenna structure for the higher band is obtained by

Fig. 2. Antenna prototype.

Fig. 3. Measured reflection coefficients for the single antenna.

the combination of the smaller bowtie, the bigger bowtie, andthe director. For the lower band, the quasi-Yagi structure con-sists of the ground plane, the bigger bowtie with a quarter-wave-length (at the center frequency of the lower band) radius, and thesmaller bowtie, which act as the reflector, the driven element,and the director, respectively. Thus, with the double quasi-Yagistructures, the antenna has directional pattern in dual bands.A prototype is manufactured on a 1-mm-thick FR-4 substrate

for 2.4-GHz (24002484-MHz)/5-GHz (51505850-MHz)WLAN. This low-cost antenna is much smaller than the similardesign in [13]. The dimension of the antenna is 26 60 mmas shown in Fig. 2. W1, W2, W3, R1, R2, R3, L1, L2, L3,L4, and L5 are 2.50, 2.46, 12.5, 6, 17.2, 6.5, 5, 2, 7, 3, and3 mm, respectively. Fig. 3 shows the measured reflectioncoefficients of the antenna, which fully covers the 2.4-GHzband and the 5-GHz band. The bandwidth of the antenna overthe 2.4-GHz band and 5-GHz band is 5.76% (140 MHz) and17.27% (960 MHz), respectively.

B. Dual-Band Dual-Polarized Antenna ArrayA compact dual-band dual-polarized antenna array for anti-

interference MIMO WLAN applications, which supports threedata streams, is presented. The array comprises six antennas forhorizontal polarization and six antennas for vertical polariza-tion as shown in Fig. 4. In order to implement MIMO and beamswitching flexibly, six antennas (H1H6) are placed in a sequen-tial, rotating arrangement on the horizontal substrate panel with

ZHENG et al.: DUAL-BAND DUAL-POLARIZED COMPACT BOWTIE ANTENNA ARRAY FOR ANTI-INTERFERENCE MIMO WLAN 239

Fig. 4. Antenna array geometry.

an equal inclination angle of 60 to form a symmetrical struc-ture. The other six antennas (V1V6) are inserted through slotsmade on the horizontal substrate panel. The central space of thehorizontal substrate panel could be used to construct an RF cir-cuit. This configuration not only keeps the full broadside 360coverage on the azimuth plane for the WLAN band, but alsooffers a dual-linear polarization to decrease the correlation be-tween different streams in MIMO.Since the proposed antenna involves so many antennas

in such a limited space, it is inevitable to induce the severemutual coupling problems. Though the dual-linear polarizationarrangement is an efficient method to decrease the couplingbetween antennas with different polarization, the couplingbetween horizontal antennas (H1H6) still needs to be reduced.The horizontal antennas (H1H6) share the same ground plane;a certain portion of excited currents will flow to other antennas,which raises the mutual coupling. Moreover, the distance be-tween two adjacent horizontal antennas is only approximately

at lower band ( is the wavelength at the centerfrequency of the lower band in the substrate), which causes thenear-field radiation coupling. Thus, the isolation between thehorizontal antennas will be worse if no strategies are appliedhere. The proposed isolation structure for horizontal antennasat lower band consists of six pairs of meandered slits. Unlikeother decoupling strategies such as adding a strip resonatorbetween the antennas [14] or incorporating a neutralizationline in between the antennas [15], the proposed slits are easyto achieve high isolation in our array and occupy little space.Each meandered slit has a configuration as shown in Fig. 5. Themeandered slits work as radiating slots. It is effective to makethe surface current at lower band converge around the slits andthus mitigate the coupling.The specialty of the proposed array is that it could support the

beam switching technique so as to mitigate interferences in anMIMO scenario. In the 3 3 (three receive and three transmitantennas) anti-interference MIMO system we proposed, thereare numerous possible antenna combinations for MIMO andbeam switching. For instance, the three groups (H1, H4, V2,V5), (H2, H5, V3, V6), and (H3, H6, V1, V4) are one kind oftypical selection for three data streams. In this case, there arefour antennas in one data stream, two for horizontal polariza-tion and two for vertical polarization. With the RF switch cir-cuits, proper antennas are selected so as to direct the beam to

Fig. 5. Meandered slits on ground plane.

Fig. 6. Prototype of the antenna array.

the desired signal and direct sidelobes or nulls to interferencesto mitigate interferences. A prototype of the array with a dimen-sion of 150 150 60 mm is shown in Fig. 6. Some designparameters related to the meandered slits are also described byFigs. 4 and 5. Detailed discussions of the array with measuredand simulated results are illustrated in Section III.

III. RESULTS AND DISCUSSION

In this section, the merits of the array are analyzed by themeasured and computed results. The measured -parameters ofthe array are firstly presented. Then, the envelope correlationcoefficient and TARC are calculated to investigate the potentialMIMO performance of the array. Finally, the radiation patternand anti-interference performance of the array are described.

A. Reflection Coefficient

The -parameters of the proposed array are measured by mi-crowave vector network analyzer E5071C, employing a coaxialcable at the desired antenna port and connecting the othersto 50- loads. Fig. 7 indicates that the horizontal antennasoperate from 2.32 to 2.54 GHz in the lower band (220 MHz,9.05% bandwidth) and from 5.00 to 5.85 GHz in the upperband (850 MHz, 15.67% bandwidth). Also, the bandwidthsof the vertical antennas are 6.21% (150 MHz) from 2.34 to2.49 GHz and 16.07% (900 MHz) from 5.15 to 6.05 GHz, asshown in Fig. 8. The bandwidth of the array ensures that thesystem could cover the 2.4-GHz (24002484-MHz) and 5-GHz(51505850-MHz) bands for WLAN.


Fig. 7. Reflection coefficients of the horizontal antennas.

Fig. 8. Reflection coefficients of the vertical antennas.

Fig. 9. Coupling between the horizontal antennas.

Fig. 10. Coupling between the vertical antennas.

B. Isolation

Isolations between antennas are important factors for anti-in-terference MIMOWLAN. Due to the symmetry of the array, theisolation between any two of the 12 antennas is sufficiently indi-cated by , , , , , ,

, , and . The measured -parameters inFigs. 911 indicate that all the isolations between the antennasare above 20 dB.

Fig. 11. Coupling between the horizontal and vertical antennas.

Fig. 12. Current distribution of the reference array (H1 is excited).

Fig. 13. Current distribution of the proposed array (H1 is excited).

Traditionally, the coupling always derives from the near-fieldradiation and current flow on the ground plane. In an arraywithout proposed slits, the coupling mainly stems from a cer-tain portion of the excited surface currents on the ground plane,especially at 2.4-GHz band for horizontal antennas. When H1is excited, the behavior of the surface currents on the groundplane in a reference array (without slits) could be illustrated bythe current distribution as shown in Fig. 12. From Fig. 12, it isnoticed that a certain portion of the excited surface current flowsto other antennas. The coupling caused by this behavior couldbe effectively reduced by incorporating themeandered slits. Theproposed meandered slit as shown in Fig. 6 has a total length of21 mm ( at 2.44 GHz) and a slit width of 0.5 mm. This21-mm-length ground slit acts as a resonator that is equivalentto a series resonator. At resonant frequency, the resonatorextracts the substrate ground current related to mutual coupling.The surface current distribution on the ground plane of the an-tenna array with slits is shown in Fig. 13. It is noticed that thecurrent that flows to other antennas is trapped around the slitswhen H1 is excited. Therefore, the isolation between two portsis enhanced. In Fig. 14, it describes results of the coupling be-tween antennaH1 andH2 in an arraywith andwithout themean-


Fig. 14. Coupling between the H1 and H2.

Fig. 15. Coupling between the H1 and H3.

dered slits. It is noticed that the of the array can be as highas about 14 dB over the 2.4-GHz band in an array withoutslits. Fig. 15 also describes a comparison of the coupling be-tween the H1 and H3. According to the contrast of the couplingsas shown in Figs. 14 and 15, the highest in-band coupling (at2.4-GHz band) is approximately 30 dB. Thus, approximately15 dB port isolation is improved by meandered slits at 2.4 GHzbetween the horizontal antennas.

C. TARC

The scattering matrix does not accurately characterize theradiating efficiency and bandwidth of a multiport antennaarray [16]. TARC must also be considered [17][19]. TARCcan be considered as a measure of the MIMO array radiationefficiency for a multiport antenna and accounts for both cou-pling and random signals combining [16]. We use TARC ratherthan the traditional scattering matrix to evaluate the radiatingefficiency and bandwidth of an antenna array. TARC is definedas the ratio of the square root of total reflected power divided

Fig. 16. Calculated TARC of three antennas with and without proposed slits.

by the square root of total incident power [16]. The TARC atthe -port antenna array can be described as

(1)

In the proposed 3 3 anti-interference MIMO array, thereare numerous combinations for data transmission. Here, threegroups (H1, H4, V2, V5), (H2, H5, V3, V6), and (H3, H6, V1,V4) are considered as an example to compute the TARC. ATARC for three antennas (H1, H2 and H3) is computed by (2)at the bottom of the page, where is the random phase angleof port excitation.The calculated TARC of the array with and without slits is

shown in Fig. 16. It is noticed that the calculated TARCwith slitsis lower than 10 dB at 2.4-GHz band and 5-GHz band. How-ever, the TARC without slits is higher than 10 dB in 2.4-GHzband and in most of the 5-GHz band. Thus, the computed TARCresults show that our array has a good radiating efficiency andlow mutual coupling so as to improve the MIMO performance.

D. Envelope Correlation CoefficientThe correlation coefficient is an important MIMO perfor-

mance metric, as it quantifies the capability of the MIMOchannel to provide parallel subchannels, which facilitates goodcapacity performance. It is associated with the loss of spec-tral efficiency and degradation of performance of an MIMOsystem [20]. The correlation coefficient is usually computedfrom radiation patterns. Considering the complex calculationprocedure, recent research indicates that for uniform signalpropagation environments, the correlation coefficient and

(2)


Fig. 17. Correlation coefficients of the (H1, H2) and (H1, H3).

Fig. 18. Correlation coefficients of the (H1, V1) and (V1, V2).

envelop correlation coefficient can be easily derived from the-parameters [20][22] with the following expression:

(3)

where and denote correlation coefficient and envelop cor-relation coefficient, respectively.Based on the measured -parameters, the envelope correla-

tion coefficient of two ports of the array is less than 40 dBin the band of 2.42.5 GHz, and less than 60 dB in the bandof 5.06.0 GHz. The low envelope correlation coefficients inWLAN band indicate a good support for MIMO performanceand demonstrate that our decoupling strategies are effective.In addition, the longer the distance between two ports, the

lower envelope correlation the results indicate. Thus, envelopecorrelation coefficients in Figs. 17 and 18 are sufficient for eval-uating all the envelope correlation coefficients of the array.

E. Radiation PerformanceThe radiation characteristics of the array are obtained by the

two-axis dual-polarization pattern measurement system. In thissection, only the results of one horizontal antenna (H1) and onevertical antenna (V1) are reported since the array has a symmet-rical arrangement. The measured and simulated radiation pat-terns at the operating band center frequency, 2.44 and 5.5 GHz,are shown in Figs. 1926. A good agreement is noticed, whichfurther verifies the simulation results by HFSS. The measured3-D radiation patterns are presented in Figs. 2831.

Fig. 19. Measured and simulated radiation pattern at 2.44 GHz in the E-plane(H1).

Fig. 20. Measured and simulated radiation pattern at 2.44 GHz in the H-plane(H1).

Fig. 21. Measured and simulated radiation pattern at 5.5 GHz in the E-plane(H1).

According to the measured results, the front-to-back ratioof the horizontally and vertically polarized arrays is approxi-mately 12 dB at 2.44 GHz and approximately 15 dB at 5.5 GHz,respectively. The vertical and horizontal antennas exhibit lowcross-polarization levels in both 2.44 and 5.5 GHz. The mea-sured results indicate a good directivity of antennas. Becauseof this directional characteristic of the array, it is facilitative to


Fig. 22. Measured and simulated radiation pattern at 5.5 GHz in the H-plane(H1).

Fig. 23. Measured and simulated radiation pattern at 2.44 GHz in the E-plane(V1).

Fig. 24. Measured and simulated radiation pattern at 2.44 GHz in the H-plane(V1).

make the null or the sidelobe of the radiation pattern direct tointerferences.The single horizontal or vertical antenna presents a fixed end-

fired main beam in the -plane. The E-plane beamwidth ofthe single horizontal or vertical antenna is about 60 , while theH-plane beamwidth is about 120 . The E-plane beamwidth of

Fig. 25. Measured and simulated radiation pattern at 5.5 GHz in the E-plane(V1).

Fig. 26. Measured and simulated radiation pattern at 5.5 GHz in the H-plane(V1).

Fig. 27. Azimuth plane coverage with H1, H4, V2, and V5 at 2.44 GHz.

one of these antennas is narrower than the H-plane. The nar-rower beamwidth in E-plane can promote distinguishing the de-sired signal from interferences, while the wider beamwidth inH-plane could improve the coverage area in elevation plane.The E-plane radiation patterns of the horizontal antennas andthe H-plane radiation patterns of the vertical antennas ensurethe coverage of the whole azimuth plane. For instance, Fig. 27describes an example of azimuth plane coverage with H1, H4,V2, and V5 at 2.44 GHz. The four antennas cover the whole


Fig. 28. Measured 3-D radiation patterns at 2.44 GHz for horizontal antennas.

Fig. 29. Measured 3-D radiation patterns at 5.5 GHz for horizontal antennas.

Fig. 30. Measured 3-D radiation patterns at 2.44 GHz for vertical antennas.

Fig. 31. Measured 3-D radiation patterns at 5.5 GHz for vertical antennas.

-plane with a crossover depth of 3 dB. With the RF switchcircuits, proper antennas could be selected so as to direct thebeam to the client to avoid interferences. If the scenario is in

Fig. 32. Layout of the measurement site.

a building, the proposed array not only covers the floor in az-imuth plane, but also covers the clients in other adjacent floorsdue to the wide beamwidth in H-plane of antennas. In addition,the measured average gains of the array are 4.5 dBi over the2.4-GHz band and 5 dBi over the 5-GHz band, respectively.

F. Anti-Interference Performance

The performance of anti-interference of the array is simplyevaluated by an experiment similarly to that in [23]. The anti-in-terference property of the array is investigated by the SIR forchanging of the interference direction. The SIR is equivalentto the interference suppression. If the receiving antenna is om-nidirectional, the SIR of the receive antenna is 0 dB when thedesired and interfering signals have equal transmitting power.Since the proposed array is directional, when the main lobe ofthe antenna directs to the desired signal and sidelobe or nulls tointerferences, the SIR will be obviously improved, and the in-terference is effectively suppressed.An indoor laboratory was chosen as the measurement envi-

ronment, as shown in Fig. 32. The array was installed at thecenter of the test site (point A) as a receiving antenna. The outputsignal of the receiving antenna was obtained by the spectrumanalyzer linked with a low noise amplifier (LNA). The desiredand interfering signals coming from various directions were as-sumed by placing the transmitting antenna around the receivingarray with a distance of 1 m (point A to point B-G). The trans-mitting antenna was a horn antenna with linear polarization. Thetransmitting power of 0 dBm was applied for both of the de-sired and interfering signals. The directions of the interferencewere given by the directions of 45 (C), 135 (E), 225 (F), and315 (G).Since there were no switches in the prototype of the array, the

desired signal was assumed to be in 0 from the point B, and H1was selected to be a receiving antenna in the array. When thedesired signal transmitted from B with horizontal polarization,the received power of H1 was 5.83 and 11.67 dBm at 2.44and 5.5 GHz, respectively.Table I shows the measured received power of H1 when the

interference came from different directions with different linearpolarizations. From Table I, it is found when the interferencewith different polarization was close to the desired signal suchas in location C or G, at least 6 dB SIR discrepancy is achieved


TABLE IMEASURED RECEIVED POWER OF H1 WHEN THE INTERFERENCE CAME FROM

DIFFERENT DIRECTIONS WITH DIFFERENT LINEAR POLARIZATIONS

TABLE IIMEASURED RECEIVED POWER OF V2 WHEN THE INTERFERENCE CAME FROM

DIFFERENT DIRECTIONS WITH DIFFERENT LINEAR POLARIZATIONS

at 2.44 GHz. This discrepancy is mainly attributed to the hor-izontal polarization characteristic of the H1, which suppressesthe interference with vertical polarization.When the interferences were in the backlobe or the sidelobe

areas such as locations E and F, the SIRs were at least 16 dB.The directional characteristic of radiation pattern ensured thatinterferences were effectively suppressed.Moreover, in another case, the desired signal was assumed

to be in 90 from the point D with vertical polarization, and V2was selected to be a receiving antenna in the array. The receivedpower of V2 was 3.50 and 16.67 dBm at 2.44 and 5.5 GHz,respectively. Table II shows the measured received power ofinterference and SIR. The main difference between these twoexperiments is that the former presented a higher SIR when theinterference was close to the desired signal. The possible reasonfor this is the difference of radiation pattern in H1s E-planeand V2s H-plane. Therefore, different quantity of power willbe received with different gains.

IV. CONCLUSION

A dual-band dual-polarized compact bowtie dipole antennaarray is proposed for anti-interference MIMO WLAN applica-tions. In the array, there are 12 antennas, six for horizontal polar-ization and six for vertical polarization. Six antennas are placedin a sequential, rotating arrangement on a horizontal substratepanel with an equal inclination angle of 60 to form a symmet-rical structure for horizontal polarization, while the other six an-tennas for vertical polarization are inserted through slots madeon the horizontal substrate panel. It is flexible to support MIMOand beam switching.

In order to diminish the size of the antenna array, a simplefeeding structure is applied to reduce the dimension of the singleantenna. The feeding part of the antenna comprises a PSL, awideband transition from the microstrip line to a PSL, and aground plane. Moreover, in the array, because of the dual linearpolarizations, more antennas could be arranged in a compactspace, which also contributes greatly on diminishing the size ofthe array.Furthermore, six pairs of meandered slits are introduced to

reduce the mutual coupling between the antennas in the lowerband. The decoupling mechanism is analyzed with the currentdistribution of the array. Meanwhile, the dual linear polariza-tions arrangement is beneficial to reduce the coupling betweenthe antennas.The anti-interference characteristic of the array is mainly due

to the radiation pattern performance of the 12 sector antennas,such as directivity, high front-to-back ratio ( ), and lowcross-polarization level. Each compact antenna in the arrayhas a directional radiation pattern due to its double quasi-Yagistructure. The E-plane beamwidth of one of these antennasis narrower than the H-plane. The narrower beamwidth inE-plane is useful for mitigating interferences, while the widerbeamwidth in H-plane could expand the coverage area inanother dimension. High of the array is valuable formitigating interferences.The prototype of the compact array has a dimension of

150 150 60 mm . The measured results indicate that theantenna array has the characteristics of wide bandwidth, highisolation, good front-to-back ratios, and average gains of 4.5and 5 dBi over the 2.4- and 5-GHz band, respectively. The mea-sured good radiation pattern characteristic ensures the abilityto fulfill the beam switching strategy to mitigate interferences.The MIMO performance of the array is analyzed and evalu-

ated by mutual coupling, envelope correlation coefficient, andTARC. The measured mutual coupling is at least 20 dB be-tween any two antennas in the array. Approximately 15 dBport isolation is improved by meandered slits at 2.4 GHz be-tween the horizontal antennas, which demonstrates the decou-pling strategy we proposed is effective to obtain high isola-tion. The calculated envelope correlation coefficient is less than40 dB in all cases, and the TARC is around 10 dB in the

desired frequency range. The anti-interference capability is alsoinvestigated by an indoor experiment. The SIR is efficiently im-proved when using the proposed array. According to the exper-iments, the maximum interference suppression is about 35 dB.The proposed antenna array is a possible candidate in anti-in-

terference MIMO WLAN applications, as well as other MIMOsystems such as Worldwide Interoperability for Microwave Ac-cess (WiMAX), Long Term Evolution (LTE), other mobile com-munication systems, and so on.

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Wen Chao Zheng received the B.S. degree in elec-trical engineering from Xian institute of Posts andTelecommunications, Xian, China, in 2008, and theM.S. degree in electrical engineering from WuhanResearch Institute of Posts and Telecommunications,Wuhan, China, in 2011, and is currently pursuing thePh.D. degree at Huazhong University of Science andTechnology, Wuhan, China.His research interests include microwave remote

sensing and multiband antenna design.

Long Zhang received the B.S. degree in communica-tion engineering and M.S. degree in electromagneticfields and microwave technology from HuazhongUniversity of Science and Technology, Wuhan,China, in 2009 and 2012, respectively.He is currently a Research Assistant with the

Science and Technology on Multi-Spectral Informa-tion Processing Laboratory, Huazhong Universityof Science and Technology. His research interestsinclude smart antenna, mobile terminal antennas,and microstrip antenna array.

Qing Xia Li (M08) received the B.S., M.S.,and Ph.D. degrees in electrical engineering fromHuazhong University of Science and Technology,Wuhan, China, in 1987, 1990, and 1999, respectively.He is presently a Professor with the Science

and Technology on Multi-Spectral InformationProcessing Laboratory, Department of Electronicsand Information Engineering, Huazhong Universityof Science and Technology. His research interestsinclude microwave remote sensing and deep spaceexploration, electromagnetic theory and application,

antenna array, and signal processing.

Yi Leng (M13) received the B.S. degree in elec-tronic engineering from the National University ofDefense Technology, Changsha, China, in 1999,and the Ph.D. degree in electronic science andtechnology from Huazhong University of Scienceand Technology, Wuhan, China, in 2008.He is currently a Research Associate and Vice

Director of the Electromagnetic Engineering Re-search Center, Air Force Early Warning Academy,Wuhan, China. His current research interests includemicrowave antennas, wireless communication, and

electromagnetic countermeasure.

Documents

01. Dual-Band Dual-Polarized Compact Bowtie Antenna Array for Anti-Interference MIMO WLAN - Zheng, Zhang, Li - 06646242