1032 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Low-Profile, Wideband Dual-Polarized AntennaWith High Isolation and Low Cross Polarization

Shi-Gang Zhou, Member, IEEE, Peng-Khiang Tan, and Tan-Huat Chio

Abstract—A wideband dual-polarized antenna with high isola-tion and low cross polarization is presented. A pair of orthogonalplanar printed dipoles, with a reflector ground plane, are used toobtain the two linear polarizations. Slots cut at the dipole’s cor-ners and shorted connections between the orthogonal dipoles areused to improve the isolation and cross polarization. Measuredresults show that the antenna achieves a from575–722 MHz (22.7% bandwidth). The isolation between the twopolarizations is better than 35 dB, and the cross-pol level is 30 dBbelow the co-pol level over the entire band. The overall height ofthe antenna is 80 mm, which is about at 575 MHz.

Index Terms—Directional antenna, dual polarization, low-pro-file antenna, wideband antenna.

I. INTRODUCTION

D UAL-POLARIZED wideband antennas are widelyneeded in both radar and communication systems that

needed polarization diversity, and many dual-polarized an-tennas have been reported in the literature. For low-profilerequirement, the microstrip class of antennas is the usualchoice. However, the cross polarization and isolation perfor-mance of a traditional microstrip antenna may not be goodenough for some applications. There are several techniques toimprove the isolation and cross polarization [1]–[7]. However,the antennas in [1]–[3] using an aperture-coupled feed gen-erally need a reflector plane behind the antenna to reduce theeffects of the platform and to eliminate back lobe radiation dueto the coupling aperture. This typically increases the overallantenna height. The “dual feed” technique is used in [4]–[6]to improve the isolation and cross polarization, and the intro-duction of a wideband balun improves the performance [7],at the expense of a more complicated design. The notchedantenna can achieve wide bandwidth, but the height of this typeof antenna is generally very large. In [8], a notched antennais proposed that exhibits wideband and compact properties,but further reduction in height poses a significant challenge.In [9], an antenna with good VSWR and isolation results isreported, but the antenna is relatively high and has relativelyhigh cross-polarization levels.In this letter, a wideband dual-polarised antenna comprising

planar dipoles with slot cuts along the dipole’s axis is proposed.The ends of the arms of the dipoles are also shorted together bya square ring. The height of the antenna prototype is 80 mm. Theantenna operates from 575 to 722MHzwith . The

Manuscript received June 19, 2012; accepted August 02, 2012. Date of pub-lication August 24, 2012; date of current version September 10, 2012.The authors are with the Temasek Laboratories, National University of Sin-

gapore, Singapore 117411, Singapore (e-mail: tslzs@nus.edu.sg).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.2012.2215299

measured results show good isolation and good cross-polariza-tion performance. Details of the antenna design and both theo-retical and experimental results are presented and discussed.

II. ANTENNA DESCRIPTION AND DESIGN GEOMETRY

The configuration of the proposed wideband dual-polarizedantenna element with good isolation is shown in Fig. 1. Twocrossed planar dipoles are printed on a 1.6-mm-thick FR4 sub-strate. The antenna is fed by the cables, whose outer part is con-nected to one of the dipole arms and inner part to the other armof the dipole. The outer part of the cable is also connected to theground, which will connect one arm of the dipole to the ground.There is another metal post at the symmetrical place with thecable connecting the other arm of the dipole to the ground. Theouter part of the cable and the shorting metal post will producea balun, which make the feed of the antenna good. The profileof the dipole edge is expressed by

(1)

The slot cut on the axis of each dipole arm is to guide the flowof the current on the dipole, which is a key to the low cross-po-larization performance. Another key characteristic of this an-tenna is a square ring that shorts the outer edges of the arms ofthe dipoles together. As the ends of the dipoles are connected, ashorted slot transmission line is formed by the adjacent edges ofthe dipole arms. By suitably tuning parameters of this shortedslot transmission line, an open circuit is presented at the dipole’sfeeding point. This suppresses the current at the feeding point,thereby improving the isolation between the two polarizations.The suppression of the current at the center part also contributesto the reduction of the antenna height.

III. RESULTS AND DISCUSSIONS

To analyze impedance and pattern, the proposed antenna wasfirst simulated by the electromagnetic (EM) full-wave simulatorHFSS. An antenna prototype is also fabricated for verification.The optimized parameters of the antenna in accordance with theconfiguration in Fig. 1 are given as follows: mm,

mm, mm, mm, mm.The antenna photograph is shown in Fig. 2.The antenna prototype is also measured using the Agilent

E8362B vector network analyzer. Fig. 3 presents the simulatedand measured VSWR and isolation results. The simulated andmeasured VSWR and isolation results of the proposed antennaare shown in Fig. 3. The results show that the antenna has about22.7% bandwidth for . The isolation between thetwo input ports of the dual-polarized antenna is better than 35 dBover the entire bandwidth. The measured and simulated VSWRagree very well with each other. However, for isolation results,

1536-1225/$31.00 © 2012 IEEE

ZHOU et al.: LOW-PROFILE, WIDEBAND DUAL-POLARIZED ANTENNA WITH HIGH ISOLATION AND LOW CROSS POLARIZATION 1033

Fig. 1 Geometry of proposed antenna. (a) 3-D view. (b) Top view. (c) Sideview.

Fig. 2. Photograph of proposed antenna.

the agreement is not as good, but the trend is similar. Consid-ering that coupling between the two ports is very weak and thevalue is very small (the simulated coupling energy is less than1/(10 000) of the radiation energy), even a very small error willproduce more than 10 times difference. The discrepancies maybe due to the fabrication tolerance, especially because the cableinners at the feeding points for orthogonal polarizations cannotbe placed exactly orthogonally.Themeasured and simulated normalized E-plane andH-plane

radiation patterns of the antenna, at frequencies of 575, 640, and

Fig. 3. Simulated and measured (a) VSWR and (b) isolation of proposedantenna.

720 MHz, are plotted in Fig. 4. Because the simulated patternsfor different port feeding are exactly same, only the simulatedpatterns when port 1 is fed are given for comparison. It showsthat the radiation patterns have good symmetry, and the crosspolarization at all frequencies is better than 30 dB. The co-polsimulated and measured patterns agree very well. Due to thesame reason as the isolation, the cross-pol simulated and mea-sured patterns do not agree with each other very well. The sim-ulated and measured realized gain of the antenna is shown inFig. 5, which shows the realized gain varies from 8.0 to 8.7 dBiover the frequency band. The simulated and measured resultsagree well, except for some small jitter over the measured gaincurve, which may be caused by measurement error.The effect of the square ring that shorts the edge of dipole

arms can be seen by looking at the simulated current plots onthe dipoles. Fig. 6 gives the current distribution (at 640 MHz)on the surface of the antenna radiating element when one of theports is excited. It can be seen that the current density at thefeeding point with the end connections is much weaker than thatwithout end connections.The effect of varying , and as shown in Fig. 1 is

studied via simulations. In all cases, other parameters are heldconstant while one is varied. Fig. 7 depicts the effect of varyingthe on the VSWR. It is seen that the second resonant fre-quency of the antenna moves higher as decreases. By judi-ciously selecting , a compromise between good VSWR andbandwidth is obtained.

1034 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 4. Measured and simulated radiation patterns of proposed antenna.

Fig. 5. Measured gain of proposed antenna.

The effect of on the VSWR is shown in Fig. 8. It is seenthat this parameter affects the bandwidth much, but influencesthe impedance match within the passband.

Fig. 6. Simulated current distribution on the surface of the antenna. (a)Withoutconnection. (b) With connection.

In Fig. 9, the length of the slot at the end of the dipole ( )is varied. It is noted that the first resonant frequency of the an-tenna becomes lower as increases, thereby increasing thebandwidth, but at the expense of higher VSWR in the midband.

ZHOU et al.: LOW-PROFILE, WIDEBAND DUAL-POLARIZED ANTENNA WITH HIGH ISOLATION AND LOW CROSS POLARIZATION 1035

Fig. 7. Simulated VSWR, varying .

Fig. 8. Simulated VSWR, varying .

From the above parameter study, we can know andmainly affect the resonant properties of the antenna. Because therepresents the length of the slots in the diagonal directions, it

affects the longest resonant mode, which means the lowest-fre-quency resonant mode. While the represents the length ofthe slot in the direction of the highest-frequency resonant cur-rent, it affects the highest-frequency part more. affects theimpedance less, but it cannot be too long, which means the con-nected line is too narrow and destroys the match of the lowestfrequency part. It cannot be too short, otherwise it will destroythe higher-frequency resonant mode.

IV. CONCLUSION

A wideband dual-polarized antenna with good isolation andcross polarization has been designed, fabricated, and tested. Pa-

Fig. 9. Simulated VSWR, varying .

rameters of the antenna are also studied. The proposed antennaachieves 22.7% bandwidth for despite the overallheight being only at the lowest operating frequency. Theisolation between the two ports is better than 32 dB, and thecross polarization is better than 30 dB over the whole fre-quency band. It has a relatively flat measured gain of about8.0 dB over the frequency band mentioned above.

REFERENCES

[1] S. K. Padhi, N. C. Karmakar, Sr., C. L. Law, and S. Aditya, Sr, “A dualpolarized aperture coupled circular patch antenna using a C-shapedcoupling slot,” IEEE Trans. Antennas Propag., vol. 51, no. 12, pp.3295–3298, Dec. 2003.

[2] S. Gao, L. W. Li, M. S. Leong, and T. S. Yeo, “A broad-band dual-po-larized microstrip patch antenna with aperture coupling,” IEEE Trans.Antennas Propag., vol. 51, no. 4, pp. 898–900, Apr. 2003.

[3] T. W. Chiou and K. L. Wong, “Broadband dual-polarised single mi-crostrip patch antenna with high isolation and low cross polarisation,”IEEE Trans. Antennas Propag., vol. 50, no. 3, pp. 399–401, Mar. 2002.

[4] H. Wong, K. L. Lau, and K. M. Luk, “Design of dual-polarizedL-probe patch antenna arrays with high isolation,” IEEE Trans.Antennas Propag., vol. 52, no. 1, pp. 45–52, Jan. 2004.

[5] T. W. Chiou and K. L. Wong, “A compact dual-band dual-polarizedpatch antenna for 900/1800-MHz cellular systems,” IEEE Trans. An-tennas Propag., vol. 51, no. 8, pp. 1936–1940, Aug. 2003.

[6] C. Y. D. Sim, C. C. Chang, and J. S. Row, “Dual-feed dual-polarizedpatch antenna with low cross polarization and high isolation,” IEEETrans. Antennas Propag., vol. 57, no. 10, pp. 3321–3324, Oct. 2009.

[7] Y. X. Guo, K. W. Khoo, and L. C. Ong, “Wideband dual-polarizedpatch antennawith broadband Baluns,” IEEE Trans. Antennas Propag.,vol. 55, no. 1, pp. 78–83, Jan. 2007.

[8] G. M. Zhang, J. S. Hong, B. Z.Wang, G. B. Song, and P. Zhang, “Com-pact wideband unidirectional antenna with a reflector connected to theground using a stub,” IEEE Antenna Wireless Propag. Lett., vol. 10,pp. 1186–1189, 2011.

[9] Y. H. Huang, Q.Wu, and Q. Z. Liu, “Broadband dual-polarised antennawith high isolation for wireless communication,” Electron. Lett., vol.45, no. 14, pp. 714–715, Jul. 2009.