Highly Selective Bandpass Filters Using Embedded Stub-Loaded Waveguide Cavity Resonators

This letter presents a design method for the$N$th-order highly selective bandpass filters (BPFs) with quasi-elliptic response, based on air-filled waveguide cavity resonators. A pair of capacitive stubs is incorporated into each resonator, enabling the generation of$2N$independently controllable transmission zeros (TZs). Compared with a conventional directly coupled resonator, the stubs widen coupling windows between adjacent resonators, removing high-precision fabrication tolerance. An equivalent circuit model is developed to validate the method. A third-order BPF is fabricated and measured. The prototype achieves an insertion loss (IL) of 0.4 dB across the passband, a fractional bandwidth of 2.5% at a center frequency of 10 GHz, and six TZs with an out-of-band rejection level of more than −50 dB from 8.9–9.5 GHz to 10.5–11.2 GHz.