4. CONCLUSIONS A simple dual-mode DC-block semicircular-patch BPF (Fig. 3) using slot technology is presented in this letter. The slotted or- thogonally feed lines are used to obtain dual-mode characteristics, while the long slot between the semicircular patches is applied to achieve DC blocking and a strong coupling. Measurement shows that the proposed BPF exhibits two degenerate modes located at 4.90 and 5.08 GHz, respectively, and the minimum insertion loss is -1.5 dB at 5 GHz. The new dual-mode filter with DC-block function has potential future application in radio frequency, mi- crowave, and millimeter-wave circuits. ACKNOWLEDGMENTS This work was supported by Alexander von Humboldt Research Fellowship, Germany. Also, the authors thank Mr. Wolfgang Tautes and Ms. Tieying Bai, for their kind help of circuit fabrica- tion in High Frequency Group, Kiel University, Germany. REFERENCES 1. J.S. Hong and S.H. 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COMPUTATION AND OPTIMIZATION OF RESONANT FREQUENCY AND INPUT IMPEDANCE OF A COAX-FED CIRCULAR PATCH MICROSTRIP ANTENNA C ¸ ig ˘ dem Sec ¸ kin Gu ¨ rel, 1 Elif Aydın, 2 and Erdem Yazgan 1 1 Department of Electrical and Electronics Engineering, Hacettepe University, Beytepe, 06532 Ankara, Turkey 2 Department of Electrical and Electronics Engineering, Atilim University, Incek, 06836 Ankara, Turkey Received 7 February 2007 ABSTRACT: Microstrip antennas are important elements of today’s wireless communication networks. In their various applications, it is critical to provide wide band operation around the resonant frequency providing good impedance match between the antenna element and the feed. In this study, new, simple, and accurate computation of resonant frequency and input impedance of double-layered, coaxial fed circular disk microstrip antenna with air gap is performed. In the analysis, new effective radius and permittivity expressions are used in simple and modified form including modal effects. Then, a new approach is pro- posed to provide input impedance matching at resonant frequency via the application of microgenetic algorithm for wide band operation. The accuracy of theoretical calculations and impedance matching approach are presented for various structural parameters and operational modes of the antenna. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2263–2267, 2007; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mop.22675 Key words: microstrip antenna; resonant frequency; input impedance; circular patch 1. INTRODUCTION In recent wireless communication applications of microstrip an- tennas, resonant frequency, bandwidth, and input impedance are important design parameters. Circular patch microstrip antennas as important elements of such systems offer various advantages pro- viding circular polarization operation. Their main drawback is narrow bandwidth characteristics, which is considerably avoided by operating the antenna around the resonant frequency. As an alternative, double-layered structure with air gap having adjustable thickness between the substrate and the ground plane is also found to be useful in obtaining the wide band operation. For both single and double-layered structures, accurate computation of resonant frequency is an important task and takes considerable interest in literature by various authors depending on the usage of various methods and approximations [1–10]. Input impedance as another important design parameter of a microstrip antenna mainly depending on the structural parameters, operational frequency, and modal field distributions must be ac- curately determined for efficient antenna operation. At resonance, input impedance has only real part exhibiting very high sensitivity to resonant frequency. For this reason, accurate determination of input impedance around the resonant frequency is important. In literature, cavity analysis takes considerable interest for such a purpose offering simple and closed form formulas providing sat- isfactory accuracy with the usage of proper effective or dynamic structural parameters instead of the physical ones [3]. In the first part of this study, resonant frequency of double- layered circular patch microstrip antenna is accurately determined via cavity analysis, using new and simple effective permittivity and patch radius expressions including modal effects. Then, new input resistance formulation is given to improve input impedance Figure 3 Photograph of the designed dual-mode BPF DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 2263