Dual-band bandpass filters using coupled fork-shaped resonators with controllable fractional bandwidths

This article presents a novel dual-band bandpass filter with controllable fractional bandwidths for the upper passband.Two open-circuited stubs are attached to the center of a stepped impedance resonator to form a fork-shaped resonator. As one port of this resonator is on the ground plane for an odd-mode equivalent circuit, the variations of resonant frequencies are analyzed just for an even-mode excitation. From the analyses, three tentative resonant frequencies are decided, and then the dimensions of the resonators are optimized. For coupled resonators, by optimizing the overlapped lengths of outside- and center-open stubs, the desired center frequencies of the dual passbands and the tunable characteristics of the upper passband are obtained, respectively. The measured results of the proposed dual-band filters on three stages are confirmed with the simulated results. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:722–725, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26653