Published in IET Microwaves, Antennas & Propagation Received on 24th December 2007 Revised on 14th May 2008 doi: 10.1049/iet-map:20070360 In Special Issue on Asia Pacific Microwave Conference 2007 ISSN 1751-8725 Dual-mode microstrip filters with adjustable transmission zeros G.M. Eryılmaz 1 C. Karpuz 2 A. Go ¨ ru ¨ r 1 1 Department of Electrical and Electronics Engineering, Nigde University, Nigde 51245, Turkey 2 Department of Electrical and Electronics Engineering, Pamukkale University, Denizli, Turkey E-mail: agorur@ieee.org Abstract: A new type of dual-mode microstrip square-loop resonator that produces asymmetrical frequency responses is proposed. The coupling between degenerate modes of the proposed dual-mode resonator is discussed depending on the perturbation size. The effect of non-orthogonal input/output feed lines located along a straight line on the frequency response is investigated using full-wave electromagnetic simulations. Two second-order bandpass filters using this type of dual-mode microstrip resonator are designed, fabricated and measured to demonstrate the dual-mode microstrip filters exhibiting transmission zero (TZ) shifting property. The one of the second-order filters produces a filter characteristic with two TZs in the upper stopband, whereas the other one has two TZs in the lower and upper stopbands. The shifting property of one of the TZs from one side of the passband to the other side is realised by only changing the perturbation size. The fourth-order filters of this type are also investigated. Both simulated and measured results are presented. 1 Introduction Microstrip bandpass filters are generally designed utilising single- or dual-mode resonators [1]. The dual-mode microstrip resonators are attractive since a single dual-mode resonator can be employed as a doubly tuned resonance circuit, and hence the number of resonators required for a prescribed filter degree is reduced by half, resulting in a reduced-size filter structure. Consequently, the dual-mode microstrip filters have been widely used in wireless communication systems because of their advantages in microwave applications requiring high-quality narrow-band bandpass filters with features such as small size, low mass and low loss. Dual-mode microstrip resonators in the microwave literature are fed by a pair of orthogonal feed lines arranged at 908 (or 2708) to produce the two degenerate modes and to couple to each other, as shown in Fig. 1a. However, the orthogonal feed lines may not be physically suitable for many engineering applications. Instead, the feed lines oriented along a straight line can be utilised as an alternative solution [2]. In other words, the dual-mode microstrip filters may be fed by a pair of feed lines arranged at 1808 geometrically, along a straight line (B–B 0 line in Fig. 1b). In this study, the proposed filters are fed by such a non-orthogonal input/output (I/O) port separated symmetrically and spatially on the loop resonator, as compared with the conventional ones. We will show that these I/O ports lead to an asymmetrical frequency response as well as the flexibility of the circuit layout. In this paper, we present a novel dual-mode microstrip filter using degenerate modes of a dual-mode microstrip square-loop resonator, as shown in Fig. 1b. In general, the dual-mode filters produce symmetrical responses with real axis or imaginary axis transmission zeros (TZs) [3–8]. We have proposed a new type of dual-mode resonator that produces asymmetrical frequency responses. The two second-order filter samples of this type have been designed, fabricated and measured to demonstrate the dual-mode microstrip bandpass filters exhibiting TZ shifting property. One of them produces a filter characteristic with two TZs in the upper stopband and the other has two TZs in the lower and upper stopbands. Shifting one of the TZs from one side of the passband to the other one is realised by only changing the perturbation size. This allows an easy design of asymmetrical responses with improved selectivity below or above the passband by changing the perturbation size. In addition, in order to place two desired IET Microw. Antennas Propag., 2008, Vol. 2, No. 8, pp. 839–847 839 doi: 10.1049/iet-map:20070360 & The Institution of Engineering and Technology 2008 www.ietdl.org