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A DUAL-MODE WAVEGUIDE PSEUDO- ELLIPTIC FILTER WITH ZERO-SHIFTING PROPERTIES Juseop Lee, 1 Man Seok Uhm, 1 In-Bok Yom, 1 and Jeong Phill Kim 2 1 Satellite Communications RF Technology Team Electronics and Telecommunications Research Institute (ETRI) 161, Gajeong-dong, Yuseong-gu Daejeon, 305-350, South Korea 2 Chung-Ang University 221, Heukseok-dong, Dongjak-gu Seoul, 156-756, South Korea Received 29 July 2005 ABSTRACT: This paper describes the design and realization of dual- mode waveguide filters with zero-shifting properties. The transmission zero can be shifted from one side of the passband to the other by simply changing the resonant frequencies of the resonators. Without changing the physical dimensions of the resonators and coupling irises, transmis- sion zeros can be shifted by adjusting the tuning and coupling screws. The filters are realized and the measured results show good agreement with the simulated ones. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 47: 357–359, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21169 Key words: dual-mode filter; zero-shifting property; transmission zeros; waveguide filter 1. INTRODUCTION With the advantage of high-frequency selectivity, elliptic and pseudo-elliptic filters have been largely applied to modern space and terrestrial communication systems. The synthesis and design method of waveguide pseudo-elliptic waveguide filters presented by Williams and Atia [1, 2] are well known, and many other related design techniques have been reported. These conventional filters use coupling schemes which always include direct coupling and cross coupling [see Fig. 1(a)]. Conventional elliptic and pseu- do-elliptic filters do not allow expedient shifting of transmission zeros from one side of the passband to the other. As the frequency allocation of communication systems have become more sophisticated, filters with asymmetric rejections have been required. Recent articles [3–5] have reported asymmetric rejection filters and some filters have exhibited zero-shifting prop- erties. Some selected direct couplings are not present in the cou- pling schemes for filters with zero-shifting properties. To the authors’ knowledge, these coupling schemes for filters with zero- shifting properties have not been implemented in dual-mode waveguide filters of orders higher than two. A possible structure for a four-pole dual-mode filter was proposed (without implemen- tation) in [3]. This paper focuses on the realization aspect of the four-pole dual-mode filter with zero-shifting properties and, consequently, gives a proof of [3] by implementing the filter. Furthermore, we demonstrate that the transmission zero can be shifted from one side of the passband to the other by adjusting the screws and without changing the size of the cavities and coupling irises, which is the advantage of this filter from a manufacturing point of view and was not dealt with in previous works. 2. DESIGN Figure 1(b) shows coupling schemes of one of four-pole pseudo- elliptic filters with zero-shifting properties. This scheme was re- MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 47, No. 4, November 20 2005 357