all nearly more than 25 dB in the band of interest. In order to verify the validity of the theoretical expectation, one prototype at the Ka-band has been fabricated and measured; all the data have also been illustrated in Figures 5 and 6. Obviously, good agreement has been observed between the simulation and the experiment; espe- cially, the high isolation between the two output ports is quite remarkable—more than 25 dB in the band of interest. The deriva- tion of measured output power is larger (about 0.45 dB) than the simulation results in the upper-frequency range, although the VSWR is better. In addition, the measured isolations S 41 and S 23 have some fluctuation in the band (especially the upper part), which is probably due to the fact that the measured results include the contribution of the microstrip transition, which will inevitably affect the isolation and the output power ratio. In addition, the measured return loss is worse than the simulation due to the microstrip transition. Undoubtedly, some optimization is still re- quired to obtain the optimum results. 5. CONCLUSION In this letter, an H-plane hybrid ring constructed in substrate- integrated rectangular waveguide (SIRW) has been presented and some design techniques have been introduced. Good performances have been observed from the simulation and experiments, espe- cially high isolation between the two output ports. Compared with the rectangular waveguide hybrid ring, this structure takes advan- tage of low cost, low profile, small volume, and ease of integration. However, the bandwidth is narrower than that of the microstrip hybrid rings although the Q factor of the SIRW hybrid ring is higher; some improvements are still required to increase the band- width. In any case, with the features of a planar structure and low profile, this device is expected to be incorporated in designs of hybrid microwave integrated subsystems. ACKNOWLEDGMENT The authors would like to express their gratitude for the financial support of the National Science Foundation of China under grant no. 60471025 and the Natural Science Foundation of Jiangsu Province under grant no. BK2004135. REFERENCES 1. D.M. Pozar, Microwave engineering, 2 nd ed., Wiley, New York, 1998. 2. J. Hirokawa and M. Ando, Single-layer feed waveguide consisting of posts for plane TEM wave excitation in parallel plates, IEEE Trans Antennas Propagat AP-46 (1998), 625– 630. 3. D. Deslandes and K. Wu, Integrated microstrip and rectangular waveguide in planar form, IEEE Microwave Wireless Compon Lett 11 (2001), 68 –70. 4. K. Wu, Integration and interconnect techniques of planar and nonplanar structures for microwave and millimeter-wave circuits-current status and future trend, Proc Asia-Pacific Microwave Conf, Taipei, Taiwan, R.O.C. (2001), 411– 416. 5. Y.L. Zhang, W. Hong, K. Wu, et al., Novel substrate integrated waveguide cavity filter with defected ground structure, IEEE Trans Microwave Theory Tech MTT-53 (2005), 1280 –1287. 6. Y. Cassivi and K. Wu, Low-cost microwave oscillator using substrate integrated waveguide cavity, IEEE Microwave Wireless Compon Lett 13 (2003), 48 –50. 7. Y. Li, W. Hong, T.J. Cui, K. Wu, et al., Simulation and experiment on SIW slot array antennas, IEEE Microwave Wireless Compon Lett 14 (2004), 446 – 448. 8. Y. Cassivi, L. Perregrini, P. Arcioni, M. Bressan, K. Wu, and G. Conciauro, Dispersion characteristics of substrate integrated rectangular waveguide, IEEE Microwave Wireless Compon Lett 12 (2002), 333–335. © 2006 Wiley Periodicals, Inc. CIRCULAR MICROSTRIP ANTENNA WITH A SECTOR-SLOT FOR DUAL- PORT OPERATION Deepti Das Krishna, C. K. Aanandan, P. Mohanan, and K. Vasudevan Centre for Research in Electromagnetics and Antennas Department of Electronics Cochin University of Science & Technology Cochin 682022, India Received 25 August 2005 ABSTRACT: Design of a dual-port circular patch antenna with a sec- tor-slot for dual-frequency operation is presented. The antenna reso- nates at two distinct frequencies with orthogonal polarizations and broad radiation characteristics. Unlike the conventional circular patch, this antenna can be microstrip-fed to operate at either of the reso- nances. The two polarizations can be simultaneously excited using two electromagnetically coupled ports with an isolation better than -30 dB between the ports. This antenna has the added advantage of size reduc- tion of 44% compared to the conventional circular patch without any reduction in gain. © 2006 Wiley Periodicals, Inc. Microwave Opt Figure 1 Geometry of the microstrip-fed circular patch antenna with a sector slot Figure 2 Measured and simulated return losses ( S 11 ) for the microstrip fed circular patch antenna with a sector slot R = 17.5 mm,  = 20°,  r = 4.36, and h = 1.6 mm DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 48, No. 3, March 2006 505