IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014 197

Wideband Circularly Polarized Cavity-BackedAperture Antenna With a Parasitic Square Patch

Wenwen Yang, Student Member, IEEE, and Jianyi Zhou, Member, IEEE

Abstract—A technique to design a wideband circularly polar-ized (CP) cavity-backed aperture antenna is presented in thisletter. The proposed antenna consists of an aperture antenna, alow-profile backed cavity, and a parasitic patch. Bidirectionalradiation of the aperture antenna is changed to unidirectionalradiation using the low-profile backed cavity. The parasitic patchis adopted to provide a favorable axial-ratio (AR) bandwidth. Theproposed antenna combines the attractive features such as wideimpedance and AR bandwidths, compact size, high aperture effi-ciency, as well as easiness of design, manufacture, and integration.The antenna operates at 6-GHz band with the overall volume of

. Measured results show that the antennaachieves a 10-dB impedance bandwidth of more than 70% anda 3-dB AR bandwidth of 43.3% with a peak gain of 8.6 dBi.

Index Terms—Aperture antenna, cavity-backed, circularly po-larized, wideband.

I. INTRODUCTION

C IRCULARLY polarized (CP) antennas have been widelyused in wireless communications such as satellite,

radar,and radio frequency identification (RFID) systems dueto its inherent advantage of insensitivity to depolarization.However, they usually suffer from narrow impedance andaxial-ratio (AR) bandwidths. Many design methods have beeninvestigated to achieve wideband CP antennas [1]–[8]. Theprinted aperture antennas are commonly used as they canprovide wideband bidirectional CP radiation [1], [2], but theyexhibit low radiation gain due to the bidirectional radiationproperties. The CP aperture antennas backed by ametal reflectoror cavity are proposed to offer unidirectional radiation [3]–[5].As discussed in [4] and [5], the low-profile reflector or backedcavity with height of less than deteriorated the CPbandwidth of the antenna. In order to solve the problem, awideband CP aperture antenna backed by an artificial magneticconductor (AMC) reflector in place of the conducting metalcavity that demonstrates a 33.2% AR bandwidth and a 36.2%impedance bandwidth is proposed by Agarwal et al. [6]. How-ever, the introduction of the AMC structure makes the designprocedure complicated, and the gain decreases rapidly in high

Manuscript received November 07, 2013; revised December 26, 2013; ac-cepted January 02, 2014. Date of publication January 09, 2014; date of currentversion February 05, 2014. This work was supported by the National NaturalScience Foundation of China under Grant 60702163, and in part by the NationalScience and Technology Major Project of China under Grants 2010ZX03007-002-01 and 2011ZX03004-003.The authors are with the State Key Laboratory of Millimeter Waves,

Southeast University, Nanjing 210096, China (e-mail: wwyang@emfield.org;jyzhou@seu.edu.cn).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2014.2298252

frequency band. Furthermore, a broadband CP cavity-backedslot antenna array with four linearly polarized disks locatedin a single circular slot is presented in [7]. This antenna arraycan achieve an AR bandwidth of 54.5% and an impedancebandwidth of 92.1%. Nevertheless, the fabrication of the an-tenna is complicated, and the size is much larger because of theprobe feeding structure and power division network. Recently,several kinds of ring-patch CP antennas that use a parasitic ringor patch suspended on the feeding structure to enhance the ARbandwidth and gain are investigated in [8]–[10].In this letter, a novel cavity-backed aperture antenna with a

parasitic patch is proposed. The circular-shaped aperture an-tenna is optimized for achieving a wider AR bandwidth of theoverall antenna than the octagonal-shaped aperture antenna thatis described in [6]. The low-profile backed cavity is used tooffer favorable unidirectional radiation, and the parasitic patchis adopted to enhance the CP performance that is deterioratedby the low-profile backed cavity. The proposed antenna com-bines the advantages of wide impedance and AR bandwidths,high aperture efficiency, and easiness of design, manufacture,and integration with RF circuits.The following three sections constitute the main part of the

letter. The antenna geometry and design consideration are de-scribed in Section II. Section III discusses the simulated resultsand measured performances of the proposed antenna and showsthe comparison to the antenna from [6] and [7]. Section IV sum-marizes the results obtained in the letter.

II. ANTENNA ELEMENT DESIGN

The configuration of the proposed cavity-backed apertureantenna is shown in Fig. 1. The antenna consists of a parasiticpatch, a circular-shaped aperture antenna, and a low-profilebacked cavity. The proposed antenna is designed with thecenter frequency of 6 GHz. The circular-shaped aperture an-tenna with L-shaped stub feeding line is designed on an FR4

substrate for the convenience of comparison to [6]and integration with other RF devices, whereas the parasiticpatch is fabricated on a substrate of Taconic TLX ( ,loss tangent is 0.0019 at 10 GHz) to reduce the dielectric losses.For measurement convenience, an impedance transformer isintroduced to transfer the input impedance of the antenna to50 . The distance between the parasitic patch and aperture an-tenna is defined as . A low-profile square cavity of widthand height is located beneath the aperture antenna.Fig. 2 demonstrates the comparison of the octagonal-shaped

aperture and circular-shaped aperture based on numerous sim-ulations by using Ansoft HFSS. It can be found that the cir-cular-shaped aperture antenna has slightly wider AR bandwidth

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198 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014

Fig. 1. (a) Geometrical configuration of the proposed antenna. (b) Apertureantenna layer. (c) Side view of the proposed antenna.

Fig. 2. Comparison of octagonal-shaped aperture and circular-shaped aperture.

in high frequency band. However, this phenomenon becomesmore obvious as the low-profile backed cavity and parasiticpatch are added. The octagonal-shaped cavity-backed apertureantenna can achieve an AR bandwidth of 38%, while the cir-cular-shaped cavity-backed aperture antenna can achieve an ARbandwidth of 42%. Moreover, the return loss of the circular-shaped cavity-backed aperture antenna seems much better in thehigher frequency band. Hence, the circular-shaped aperture an-tenna is adopted to obtain optimal performance.The working principle of the circular-shaped aperture an-

tenna is studied first. As depicted in Fig. 3, the lower resonantfrequency is corresponding to the TE mode of the circularslot, while the higher resonant frequency is caused by the TEmode. However, the current distributions of the two resonantmodes indicate that the corresponding relations are just approx-imate equivalent since they are not strictly symmetric along thecircular slot. From the above discussion, it can be concluded

Fig. 3. Current distributions at different times offrom left to right. (a) 5 GHz. (b) 6.7 GHz.

Fig. 4. Comparison of the proposed antenna in different cases.

that the resonant frequencies of the circular-shaped aperture aremainly decided by the dimensions of and . The authorswould like to use

(1)

(where is the speed of light in free space) to calculate the res-onant frequency of TE ; the second term in the equation is thecorrection factor considering the presence of different dielectricmedia on the two sides of the aperture antenna [11]. The centerfrequency can be estimated once is obtained.The effects of the low-profile backed cavity ( at

6 GHz) and parasitic patch are also studied as shown in Fig. 4.It is seen that the AR bandwidth of the aperture antenna withouta backed cavity and parasitic patch is the widest, while the gainis quite low due to its bidirectional radiation properties. How-ever, the gain of the cavity-backed aperture antenna withoutparasitic patch is about 1 dB less than the proposed antenna inthe whole band since the AR performance is deteriorated bythe low-profile backed cavity (more than 5 dB). In other words,the parasitic patch can be used to complement the deterioratedCP performance of the aperture antenna with a low-profilebacked cavity. According to [12], the parasitic patch should bedesigned to resonant at the highest frequency in the AR band ofthe aperture antenna to obtain the widest AR bandwidth sincethe amplitudes difference of the patch’s two eigen-resonant

YANG AND ZHOU: WIDEBAND CP CAVITY-BACKED APERTURE ANTENNA WITH PARASITIC SQUARE PATCH 199

Fig. 5. Effects of varying on the return loss and axial ratio.

Fig. 6. Effects of varying on the return loss and axial ratio.

Fig. 7. Effects of varying and on the return loss and axial ratio.

currents becomes larger quickly in the band that is higher thanits resonant frequency.To evaluate the effects of the antenna dimensions on the re-

turn loss and AR, a parametric study is performed. The simu-lated return loss and AR of the proposed antenna by varyingare shown in Fig. 5. It is seen that the higher resonant fre-

quency is mainly determined by the parasitic patch, and the CPperformance in the AR band is affected by the coupling be-tween the cavity-backed aperture antenna and parasitic patch.The optimized return-loss and AR bandwidths can be achievedwhen mm ( at 6 GHz), which is similar as de-scribed in [8]. Fig. 6 shows the simulated return loss and ARwith different values of , where we can observe that the peak

Fig. 8. Photograph of the fabricated antenna.

Fig. 9. Simulated and measured return loss of the proposed antenna.

appearing in AR curve moves to the lower frequency as in-creases. This is the effect due to the internal resonance of the slotantenna loaded cavity. Therefore, the width of the cavity shouldbe adjusted to keep away from the internal resonance for an op-timal performance The variations of return loss and AR with thedepth of the cavity and width of the parasitic patch aredepicted in Fig. 7. It can be seen that good CP performance in thecentral zone of the AR band can be still maintained as long as theparasitic patch grows properly with decreasing. The achievedAR bandwidth is about 42% for mm % for

mm % for mm , and 16%for mm . However, the return loss of the an-tenna changes slightly as the parameters mentioned above arenot dramatically varied.From the discussion above, it can be learned that a favorable

CP performance can be achieved by suspending a patch abovethe low-profile cavity-backed aperture antenna that has a dete-riorative CP characteristic in itself as known in [4] and [5].

III. EXPERIMENTAL VERIFICATION

In order to validate the simulated performance of the pro-posed antenna, prototypes of the antenna are fabricated by usingprinted circuit board (PCB) process as shown in Fig. 8. Detailedgeometrical parameters of the proposed antenna are listed as fol-lows (all in millimeters):

.The simulated and measured of the proposed antenna

are plotted in Fig. 9. It can be found that the return loss is below10 dB from 4.73 to more than 9 GHz. The overall tendencies

200 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014

Fig. 10. Simulated and measured radiation pattern of the proposed antenna.(a) -plane at 5 GHz. (b) -plane at 5 GHz. (c) -plane at 7 GHz.(d) -plane at 7 GHz.

Fig. 11. Simulated and measured axial ratio and gain of the proposed antenna.

TABLE IPERFORMANCE COMPARISON AT 6 GHZ

of the simulation and measurement agree well with each other.Fig. 10 depicts the simulated and measured radiation patterns

in the - and -plane, respectively, for 5 and 7 GHz. The CPperformance was measured using a rotating linearly polarizedtransmitting horn antenna. The ripples in the pattern representthe quality of the CP radiation. As illustrated in Fig. 11, themeasured 3-dB AR bandwidth is around 43.3% (4.8–7.4 GHz),which is completely within the 2-VSWR impedance bandwidth.The measured peak gain is 8.6 dBi at 7.4 GHz, while a gain ofmore than 6 dBi in the whole AR bandwidth is achieved.The comparison between the measured performances of

the proposed antenna and antennas reported in [6] and [7] islisted in Table I. Compared to the antenna presented in [6],the proposed antenna shows larger impedance and 3-dB ARbandwidths while having slightly larger volume. On the con-trary, the proposed antenna has smaller AR bandwidth than theantenna presented in [8] while having much smaller volume.Furthermore, the proposed antenna is much easier for design,fabrication, and integration than both mentioned above.

IV. CONCLUSION

In this letter, a technique to design a wideband circularly po-larized cavity-backed aperture antenna is presented. A prototypeoperating at 6 GHz has been designed and measured to validatethe concept. The prototype achieves an impedance bandwidthof more than 70%, an AR bandwidth of 43.3%, and a peak gainof 8.6 dBi. The proposed antenna exhibits promising character-istics of wide impedance and AR band, high aperture efficiency(more than 65% except 7–7.4 GHz), and easiness of design, fab-rication, and integration.

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