A Microstrip Box-Type Coupling Bandpass Filter Using Even/odd-Symmetric Electric Field Distributions of Half-Wavelength Resonator

This paper proposes a new microstrip fourth-order box-type coupling bandpass filter (BPF) using the feature of even- and odd-symmetric electric field distributions of open-end half-wavelength resonator. To generate transmission zeros (TZs) at arbitrary frequency positions, a box-type coupling BPF using even/odd-mode half-wavelength resonators has been proposed so far. In the conventional BPF, however, metallic vias are indispensable for even-mode resonators with shorted ends. The vias degrade frequency responses, owing to a low fabrication accuracy. To solve the problem, the proposed BPF gives a new insight to TZ generation scheme using only opened resonators. The effectiveness of the proposed BPF is demonstrated by design and test of a prototype filter with center frequency 2 GHz, 5% bandwidth, and four TZs.