wideband circularly polarized patch antenna using

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 56, NO. 2, FEBRUARY 2008 319

Wideband Circularly Polarized Patch Antenna UsingBroadband Baluns

Yong-Xin Guo, Senior Member, IEEE, Kah-Wee Khoo, Student Member, IEEE, andLing Chuen Ong, Senior Member, IEEE

Abstract—A novel 90 broadband balun comprising a broad-band 90 Schiffman phase shifter is introduced as a means ofenhancing the wideband circular polarization performance ofdual-fed type microstrip antennas. The proposed 90 broadbandbalun delivers good impedance matching, balanced power splittingand consistent 90 ( 5 ) phase shifting, across a wide band-width ( 57.5%). A circular patch antenna utilizing the proposed90 broadband balun is shown to attain measured impedance(S11 10 dB) and axial ratio (AR 3 dB) bandwidths of60.24% and 37.7%, respectively, for the dual L-probe case; and71.28% and 81.6% respectively, for the quadruple L-probe case.

Index Terms—Circular polarization, microstrip antennas, wide-band antennas.

I. INTRODUCTION

CIRCULARLY polarized (CP) microstrip antennas arewidely employed in radar, navigation, satellite and mobile

communication systems. Circular polarization, compared tolinear polarization, allows for greater flexibility in orientationangle between transmitter and receiver, better mobility andweather penetration, and reduction in multipath reflections andother kinds of interference. Microstrip antennas are low profileand light weight, easy to fabricate, conformable to mountingstructures, and compatible with integrated circuit technology.However, inherent limitations include the achievable impedanceand axial-ratio bandwidths.

CP waves are produced when two or more orthogonal lin-early polarized modes, of equal amplitude and 90 phase dif-ference, are independently excited. For microstrip antennas ofthe single-fed type [1]–[5], circular polarization can be gener-ated without the need for an external polarizer. However, theallowable 3-dB axial ratio bandwidth is typically less than 10%.Notable exceptions involve the use of an L-shaped ground plane[4], or a parasitic patch element [5].

For microstrip antennas of the dual-fed type [6]–[12], circularpolarization can be generated with the use of an external polar-izer, resulting in a larger footprint beneath the patch. Comparedto the single-fed type, much wider impedance and axial-ratiobandwidths can be achieved. Feed network configurations

Manuscript received November 29, 2006; revised July 21, 2007.The authors are with the Institute for Infocomm Research, Singapore

117674, Singapore and also with the Electrical and Computer EngineeringDepartment, National University of Singapore, Singapore 117576, Singapore(e-mail: [email protected]).

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.2007.915427

Fig. 1. Schematics of the conventional 90 hybrid coupler.

comprising Wilkinson power dividers [6]–[9], a log periodicbalun [10], and a three-stub 90 hybrid coupler [11], havebeen explored. The conventional two-stub ( 25% bandwidth)or three-stub ( 40% bandwidth) branch-line hybrid couplershave been commonly used to obtain circular polarization. Aquadruple L-probe circular patch antenna utilizing a pair oftwo-stub 90 hybrid couplers was shown to deliver a measuredimpedance bandwidth of 45% and axialratio-bandwidth (AR 3 dB) of 45% [12].

In this paper, we propose the use of a novel 90 broadbandmicrostrip balun as a means of enhancing the wideband circularpolarization performance of dual-fed type microstrip antennas.The proposed 90 broadband balun delivers good impedancematching, balanced output ports power division and consistent90 output ports phase difference, over a considerably wideband ( 57.5%). We demonstrate that for both the dual andquadruple L-probe circularly polarized circular patch antennas,the use of the proposed 90 broadband balun allows for wideimpedance and axial-ratio bandwidths. The radiation patternsand gain are shown to be stable across the passband.

II. FEED NETWORK CONFIGURATION

A. Conventional 90 Hybrid Coupler

The conventional 90 hybrid coupler, commonly used as anexternal polarizer for dual-fed type CP antennas, is shown inFig. 1. This symmetrical 3-dB directional coupler provides bal-anced power splitting and 90 phase shifting between its outputports. The isolation port was terminated to a 50 resistor. Forconvenient analysis, the input and output ports of the feed net-works presented in this paper, were all set to 50 .

B. Proposed 90 Broadband Balun

The proposed 90 broadband balun, as shown in Fig. 2,delivers both balanced power splitting and regular 90 phaseshifting, across a wide band. This new balun comprises a

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320 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 56, NO. 2, FEBRUARY 2008

Fig. 2. Schematics of the proposed 90 broadband balun.

Fig. 3. Layout of C-Section coupled lines in the proposed 90 broadbandbalun.

cascade of a 3-dB Wilkinson power divider, for widebandimpedance matching and balanced power splitting, and a novelbroadband 90 Schiffman phase shifter [13], for widebandconsistent 90 phase shifting. The characteristic impedancesare given by , , and .Compared with our previous wideband CP patch antenna [14],the newly proposed feed network can have wider transmissionline widths and thus afford better fabrication tolerances.

Fig. 3 shows the layout of the C-section coupled line pair,separated by a small distance of . The gray-shadedrectangular slot, of dimensions , ,and , was cut out on the ground plane, beneaththe C-section coupled lines, to allow for the odd-mode capac-itance to decrease and the even-mode capacitance to decreaseeven faster. The 23.4 mm by 3.05 mm rectangle patch, encap-sulated by the rectangular slot, functions as a capacitor whichcompensates the odd-mode capacitance. This patterned groundplane approach provides for regular 90 phase shifting withminimal insertion losses, over a wide band. The proposed 90broadband microstrip balun is easily fabricated by photolitho-graphic process, with the network layer and patterned groundplane layer respectively printed on each side of a double-sidedsingle-layer PCB.

C. Simulated Results

For this paper, all simulations were performed using IE3D, acommercially available electromagnetic field solver based onthe method of moments (MoM). Fig. 4 shows the simulated

Fig. 4. Simulated return loss comparison between the conventional 90 hybridcoupler and the proposed 90 broadband balun.

Fig. 5. Simulated output ports amplitude response comparison between theconventional 90 hybrid coupler and the proposed 90 broadband balun.

return loss comparison between the two external polarizers.The 90 broadband balun exhibits a wide impedance band-width of 187.6%, from 0.09 to 2.81 GHz,while the regular 90 hybrid coupler exhibits a much narrowerimpedance bandwidth of 30.9%, from 1.53 to2.09 GHz. Fig. 5 shows the simulated output ports amplituderesponse comparison between the two external polarizers. The90 broadband balun exhibits balanced output ports powerdistribution [ ( 0.5 dB)] over a wide bandof 91.9%, from 0.87 to 2.35 GHz, while the regular 90 hy-brid coupler exhibits balanced output ports power distribution[ ( 0.5 dB)] over a much narrower band of14%, from 1.66 to 1.91 GHz. Fig. 6 shows the simulated outputports phase difference comparison between the two externalpolarizers. The 90 broadband balun exhibits consistent 90

output ports phase difference over a considerably wideband of 66.7%, from 1.3 to 2.6 GHz, while the regular 90hybrid coupler exhibits consistent 90 output ports phasedifference over a much narrower band of 32%, from 1.47 to2.03 GHz.

Combining the simulated results in Fig. 4, Fig. 5 and Fig. 6,it is observed that the proposed 90 broadband balun deliveredlow input port return loss , balanced output

GUO et al.: WIDEBAND CP PATCH ANTENNA USING BROADBAND BALUNS 321

Fig. 6. Simulated output ports phase difference comparison between theconventional 90 hybrid coupler and the proposed 90 broadband balun.

Fig. 7. Geometry of the proposed CP dual L-probe circular patch antenna.

ports power distribution [ ( 0.5 dB)],and consistent 90 output ports phase difference over asignificantly wide band of 57.5%, from 1.3 to 2.35 GHz; hencewe term it a “broadband” balun. The conventional 90 hybridcoupler delivered low input port return loss ,balanced output ports power distribution [( 0.5 dB)], and consistent 90 output ports phasedifference over a much narrower band of 14%, from 1.66to 1.91 GHz; inherently limited by its output port powerdistribution.

III. CIRCULARLY POLARIZED DUAL L-PROBE CIRCULAR

PATCH ANTENNA

A. Antenna Geometry and Feed Network Layout

The geometry of the CP dual L-probe circular patch antennais shown in Fig. 7. The circular copper patch, of diameter

, has an air substrate height above agrounded Rogers RO4003 dielectric substrate of thickness

and dielectric constant . The feed network,comprising the proposed 90 broadband balun, was printed onthe RO4003 substrate. The two L-probe feeds, each of diameter

Fig. 8. Simulated and measured return loss of the proposed CP dual L-probecircular patch antenna.

Fig. 9. Simulated and measured axial ratio of the proposed CP dual L-probecircular patch antenna.

, vertical length , and horizontallength , were orthogonally oriented and positioneda distance away from the circumference of thepatch, and soldered to the respective output ports of the feednetwork. The L-probe feeds excite the radiating patch elementby proximity coupling.

B. Simulated and Measured Results

Fig. 8 shows the simulated and measured return loss of thedual L-probe antenna. The proposed antenna exhibits consid-erably wide simulated and measured impedance bandwidths

of 59.52%, from 1.18 to 2.18 GHz, and60.24%, from 1.16 to 2.16 GHz, respectively. Note that theL-probe single-element rectangular patch antenna has a typicalimpedance bandwidth of around 30% [15],[16].

Fig. 9 shows the simulated and measured axial ratio ofthe dual L-probe antenna. The antenna exhibits rather widesimulated and measured 3-dB axial-ratio bandwidths of 39%,from 1.26 to 1.87 GHz, and 37.7%, from 1.25 to 1.83 GHz,respectively. Fig. 10 shows the simulated and measured gain


Fig. 10. Simulated and measured boresight gain of the proposed CP dualL-probe circular patch antenna.

of the dual L-probe antenna. The antenna exhibits a simulated3-dB gain bandwidth of 34.2%, from 1.43 to 2.02 GHz, with itshighest gain of 8.6 dBi at 1.8 GHz, and a measured 3-dB gainbandwidth of 38.6%, from 1.38 to 2.04 GHz, with its highestgain of 8.53 dBi at 1.8 GHz. It is observed that the measuredresults agree reasonably well with the simulated results.

Figs. 11 and 12, show the measured radiation patterns forthe dual L-probe antenna at 1.3, 1.6, and 1.8 GHz at the

and planes, respectively. Across this passband, itis observed that on both principle planes, the antenna exhibitsgenerally low angular axial ratio around its boresight. The slightasymmetry observed in the H- and V-polarization patterns canbe attributed to the asymmetrical feed orientation of the dualL-probe antenna configuration.

These results reveal significant enhancements in theimpedance and axial ratio bandwidths over the dual L-probeantenna presented in [12]. In terms of the common frequencycoverage of , axial ratio 3 dB, and 3-dBgain (gain 5.53 dBi), the proposed CP antenna exhibits ameasured CP bandwidth of 28.04% from 1.38 to 1.83 GHz.

IV. CIRCULARLY POLARIZED QUADRUPLE L-PROBE CIRCULAR

PATCH ANTENNA

A. Antenna Geometry and Feed Network Layout

The geometry of the CP quadruple L-probe circular patchantenna is shown in Fig. 13. The quadruple L-probe antennashares the same antenna parameters with the dual L-probe an-tenna shown in Fig. 7. The feed network, comprising a pair ofthe proposed 90 broadband baluns connected by a 180 trans-former, was printed on the RO4003 substrate. To provide 180phase shifting, the lengths of the microstrip branches must differby , where refers to the guide wavelengthat the center operating frequency, say, 1.8 GHz, in this work.The input transmission line is connected to the two microstripbranches by a quarter-wavelength transformer with character-istic impedance given by . The four L-probe feedswere soldered to the respective output ports of the balun pair, or-thogonally orientated, and provided equal amplitude power withrelative excitation phases of 0 , 90 , 180 and 270 .

Fig. 11. Measured radiation patterns at � = 0 for the proposed CP dualL-probe circular patch antenna at (a) 1.3 GHz, (b) 1.6 GHz, and (c) 1.8 GHz.

B. Simulated and Measured Results

Fig. 14 shows the simulated and measured return loss of thequadruple L-probe antenna. The antenna exhibits a considerably


Fig. 12. Measured radiation patterns at � = 90 for the proposed CP dualL-probe circular patch antenna at: (a) 1.3 GHz, (b) 1.6 GHz, and (c) 1.8 GHz.

wide simulated and measured impedance bandwidthof 73%, from 1.07 to 2.3 GHz and 71.28%, from 1.21

to 2.55 GHz, respectively. In comparison, the circular patch an-tenna also fed by four sequentially rotated proximity-coupled

Fig. 13. Geometry of the proposed CP quadruple L-probe circular patch an-tenna.

Fig. 14. Simulated and measured return loss of the proposed CP quadrupleL-probe circular patch antenna.

L-probes orientated to have relative phases of 0 , 90 , 180 and270 , but using a feed network comprising a pair of 90 hybridcouplers, delivered a 10-dB return loss bandwidth of 45% [12].Compared to the results in [12], [15], [16], it is noteworthy thatmuch wider impedance bandwidth was achieved with the pro-posed broadband feed network.

Fig. 15 shows the simulated and measured axial ratio of thequadruple L-probe antenna. The antenna exhibits simulated 3-dBand 2-dB axial-ratio bandwidths of 62%, from 1.27 to 2.41 GHz,and 48.8%, from 1.33 to 2.18 GHz, respectively. The measured3-dB and 2-dB axial ratio bandwidths are 81.6%, from 1.03 to2.45GHz,and77.7%,from1.07to2.43GHz,respectively.Fig.16shows the simulated and measured gain of the quadruple L-probeantenna. The antenna exhibits a simulated 3-dB gain bandwidthof 46.9%, from 1.34 to 2.16 GHz, with its highest gain of 8.6 dBiat 2 GHz, and a measured 3-dB gain bandwidth of 52.2%, from1.29 to 2.2 GHz, with its highest gain of 8.1 dBi at 1.8 GHz. Itis observed that the measured results agree reasonably well withthe simulated results.

Figs. 17 and 18, show the measured radiation patterns for thedual L-probe antenna at 1.2, 1.8, and 2.2 GHz at the and


Fig. 15. Simulated and measured axial ratio of the proposed CP quadrupleL-probe circular patch antenna.

Fig. 16. Simulated and measured boresight gain of the proposed CP quadrupleL-probe circular patch antenna.

planes, respectively. Across this passband, it is ob-served that on both principle planes, the antenna exhibits rathersymmetrical H- and V-polarization patterns, and generally lowangular axial ratio around its boresight.

These results reveal significant enhancements in theimpedance and axial ratio bandwidths over the dual L-probeantenna presented in the previous section and the quadrupleL-probe antenna presented in [12]. In terms of the commonfrequency coverage of , axial ratio 3 dB, and3-dB gain (gain 5.1 dBi), the proposed CP antenna exhibits amuch wider measured CP bandwidth of 52.2% from 1.29 to 2.2GHz. The impedance and gain bandwidths of the antenna areenhanced due to the use of the L-probe feeding technique anda thick air substrate. The impedance and axial ratio bandwidthsof the antenna are further enhanced due to the symmetricalsequentially rotated four point feeding structure with each feedsupplied wideband balanced power distribution and appropriatephasing.

V. CONCLUSION

We have shown that for both the dual and quadruple L-probecircularly polarized circular patch antennas, the use of the

Fig. 17. Measured radiation patterns at � = 0 for the proposed CP quadrupleL-probe circular patch antenna at (a) 1.2 GHz, (b) 1.8 GHz, and (c) 2.2 GHz.

proposed 90 broadband balun allowed for significantly im-proved impedance and axial ratio bandwidths. Our proposed


Fig. 18. Measured radiation patterns at � = 90 for the proposed CPquadruple L-probe circular patch antenna at (a) 1.2 GHz, (b) 1.8 GHz, and (c)2.2 GHz.

feed network implementation may be conceptually extended toother dual-fed type circularly polarized patch antennas.

REFERENCES

[1] H. Iwasaki, “A circularly polarized small-size microstrip antennawith a cross slot,” IEEE Trans. Antennas Propag., vol. 44, no. 10, pp.1399–1401, Oct. 1996.

[2] W. K. Lo., J. L. Hu, C. H. Chan, and K. M. Luk, “L-shaped probe-feed circularly polarized microstrip patch antenna with a cross slot,”Microw. Opt. Technol. Lett., vol. 25, no. 4, pp. 251–253, May 2000.

[3] J. S. Row, “The design of a square-ring slot antenna for circular polar-ization,” IEEE Trans Antennas Propag., vol. 53, no. 6, pp. 1967–1972,Jun. 2005.

[4] F. S. Chang, K. L. Wong, and T. W. Chiou, “Low-cost broadband cir-cularly polarized patch antenna,” IEEE Trans. Antennas Propag., vol.51, no. 10, pp. 3006–3009, Oct. 2003.

[5] K. L. Chung and A. S. Mohan, “A systematic design method to ob-tain broadband characteristics for singly-fed electromagnetically cou-pled patch antennas for circularly polarization,” IEEE Trans. AntennasPropag., vol. 51, no. 12, pp. 3239–3248, Dec. 2003.

[6] S. D. Targonski and D. M. Pozar, “Design of wideband circularly po-larized aperture-coupled microstrip antennas,” IEEE Trans. AntennasPropag., vol. 41, no. 2, pp. 214–220, Feb. 1993.

[7] D. M. Pozar and S. M. Duffy, “A dual-band circularly polarized aper-ture-coupled stacked microstrip antenna for global positioning satel-lite,” IEEE Trans. Antennas Propag., vol. 45, no. 11, pp. 1618–1625,Nov. 1997.

[8] K. L. Wong and T. W. Chiou, “Single-patch broadband circularly-po-larized microstrip antennas,” in Proc. IEEE Antennas Propag. Soc. Int.Symp. Dig., Jul. 2000, vol. 2, pp. 984–987.

[9] K. L. Lau and K. M. Luk, “A novel wide-band circularly polarizedpatch antenna based on L-probe and aperture-coupling techniques,”IEEE Trans. Antennas Propag., vol. 53, no. 1, pp. 577–580, Jan. 2005.

[10] P. H. Rao, V. F. Fusco, and R. Cahill, “Wide-band linear and circularlypolarized patch antenna using a printed stepped T-feed,” IEEE Trans.Antennas Propag., vol. 50, no. 3, pp. 356–361, Mar. 2002.

[11] X. M. Qing, “Broadband aperture-coupled circularly polarized mi-crostrip antenna fed by a three-stub hybrid coupler,” Microw. Opt.Technol. Lett., vol. 40, no. 1, pp. 38–41, Jan. 2004.

[12] W. K. Lo, C. H. Chan, and K. M. Luk, “Bandwidth enhancement ofcircularly polarized microstrip patch antenna using multiple L-shapedprobe feeds,” Microw. Opt. Technol. Lett., vol. 42, no. 4, pp. 263–265,Aug. 2004.

[13] Y. X. Guo, Z. Y. Zhang, and L. C. Ong, “Improved widebandSchiffman phase shifter,” IEEE Trans. Microw. Theory Tech., vol. 54,no. 3, pp. 1196–1200, Mar. 2006.

[14] L. Bian, Y. X. Guo, L. C. Ong, and X. Q. Shi, “Wideband circularly-polarized patch antenna,” IEEE Trans. Antennas Propag., vol. 54, no.9, pp. 2682–2686, Sep. 2006.

[15] C. L. Mak, K. M. Luk, K. F. Lee, and Y. L. Chow, “Experimental studyof a microstrip antenna with an L-shaped probe,” IEEE Trans. AntennasPropag., vol. 48, no. 5, pp. 777–783, May 2000.

[16] Y. X. Guo, C. L. Mak, K. M. Luk, and K. F. Lee, “Analysis and de-sign of L-probe proximity fed patch antennas,” IEEE Trans. AntennasPropag., vol. 49, no. 2, pp. 145–149, Feb. 2001.

Yong-Xin Guo (SM’05) received the B.Eng. andM.Eng. degrees from Nanjing University of Scienceand Technology, Nanjing, China, and the Ph.D.degree from City University of Hong Kong, all inelectronic engineering, in 1992, 1995, and 2001,respectively.

From 1995 to 1997, he was a Teaching andResearch Assistant and then a Lecturer in theDepartment of Electronic Engineering, Nanjing Uni-versity of Science and Technology. From January1998 to August 1998, he was a Research Associate

in the Department of Electronic Engineering, City University of Hong Kong,where, from December 1998 to September 2001, he was a Research Student.Since September 2001, he has been with the Institute for Infocomm Research,Singapore, where he is currently a Research Scientist. He has also heldappointments of Guest Professor and Ph.D. Student Supervisor at NanjingUniversity of Science and Technology and Adjunct Assistant Professor at theNational University of Singapore. He has published over 100 technical papersin international journals and conference proceedings. His publications havebeen cited by others over 200 times. He holds one Chinese Patent and one U.S.patent. His current research interests include microstrip antennas and dielectricresonator antennas, microwave and millimeter-wave circuits, LTCC passivesand modules, and radio-over-fiber technology for broadband communications.


He is an organizer of the workshop on radio-over-fiber technologies at theIEEE Radio and Wireless Symposium in Orlando, FL, in 2008 and was a tech-nical program committee member of the IEEE TENCON2006, IEEE ICCS2006,IEEE RFIT2007, and IEEE VTC2008 (Spring). He is a regular reviewer forthe IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, IEEE ANTENNA

AND WIRELESS PROPAGATION LETTERS, IEEE TRANSACTIONS ON MICROWAVE

THEORY AND TECHNIQUES, IEEE MICROWAVE AND WIRELESS COMPONENTS

LETTERS, RADIO SCIENCE, IET Microwave, Antennas and Propagation, Elec-tronics Letters, etc.

Kah-Wee Khoo (S’04) was born and educatedin Singapore. He received the B.Eng. and M.Eng.degrees in electrical engineering from the NationalUniversity of Singapore, in 2005 and 2008, respec-tively.

He has been with the Institute for InfocommResearch, Singapore, since 2004. His current re-search interests include printed antennas, dielectricresonator antennas and RF circuit design.

Ling Chuen Ong (SM’02) received the Ph.D. degreefrom the University of Birmingham, Birmingham,U.K., in 1996.

From 1992 to 1994, he was a Research Associatewith the University of Birmingham. From 1996 to1999, he was with Singapore Telecom as a NetworkPlanner and Project Manager for its first digitaltrunked radio system. Currently, he is an AssistantDepartment Manager with the Institute for Info-comm Research, Agency for Science, Technologyand Research (A STAR). His research interests

include radio-over-fiber technology for intelligent transport systems and futurewireless communications, low temperature co-fired ceramics and ultrawide-band technology. He is also an Adjunct Assistant Professor with the NationalUniversity of Singapore and Nanyang Technological University, Singapore.

Dr. Ong’s Ph.D. studies were financed by a Science and Engineering Re-search Council grant and a postgraduate scholarship from the Institute of Elec-trical Engineers (IEE), London, U.K.

wideband circularly polarized patch antenna using

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