This paper focuses on the realization of low spurious responses by various bandpass filters (BPFs) using open-ended λ/2 resonators. The first part of this paper gives the resonance characteristics of the open-ended λ/2 resonators when the excitation methods are chosen. Secondly, various BPFs obtained with our methodology are provided. For constructing the BPF, (1) point-coupled resonators, (2) comb-line resonators, (3) quasi comb-line resonators and (4) parallel-coupled resonators are used. It is verified that the presented BPFs can be used to obtain low spurious responses both theoretically and experimentally.

The aim of this study is to examine the effectiveness of various open-ended resonators. According to the required filter responses, the application to microwave filters based on presented open-ended resonators is systematically examined as well. First, the resonance property of the basic open-ended resonator is discussed based on even-and odd-mode analysis. The intrinsic property of a tapped open-ended resonator is also discussed here. Second, the basic properties of a stepped impedance resonator (SIR) and a loaded-element resonator are examined theoretically for improvement of spurious responses and the dual-passband response. The basic operations of these resonators are also explained based on even- and odd-mode analysis. Examples for filter applications based on presented resonators are also provided. We found that the intrinsic properties of the open-ended resonators are very useful for practical filter responses.