Modular Design of Waveguide Bandpass Filters With Improved Stopband and High Selectivity

A class of waveguide bandpass filters (BPFs) with parallel coupling topology is proposed by stacking rectangular cavities. Compared with transversal filters, each layer comprises more than one cavity, and these cavities are cascaded with electric or magnetic coupling. The stopband performance could be improved by properly adjusting the width-length ratio of rectangular cavity to push up the higher-order modes in the cavities, while the attractive property of transversal filter, i.e., modular design, is still valid and useful in simplifying the filter design process and producing transmission zeros (TZs) out of the passband. A fourth -, sixth -, and seventh-order BPF is successively designed, and frequency of the first spurious mode is increased from 1.12 to 1.48 times the operating frequency. Due to the parallel coupling topology, multiple TZs are generated at both sides of the passband, making the suppression more than 20 dB out of passband. Among these filters, the fourth- and sixth-order BPFs are fabricated, and the measured results demonstrate the effectiveness of the proposed method.