Compact metal block-loaded cavity filter with parasitic mode suppression and extended stopband

Abstract

A compact cavity bandpass filter loaded with metal blocks is proposed in this paper. Two dual-mode resonators and one single-mode resonator work together to create a quasi-ellipse response. Each resonant cavity is loaded with metal blocks at the strongest point of the electric field of resonant modes to reduce size. Although the metal block in the dual-mode cavity introduces a parasitic mode, the single-mode resonator effectively suppresses it by weakening the energy propagation of that mode. Furthermore, the position of the metal block has less influence on the ${\mathrm{TE}}_{202}$ mode in dual-mode resonator and ${\mathrm{TE}}_{201}$ mode in single-mode resonator. This means that the resonant frequencies of these modes will be much higher due to the reduction of the cavity dimensions, expanding the filter’s stopband to $2.17\times f_0$ with a suppression level better than 34 dB. An experimental prototype was fabricated and measured. The simulation and measurement results show good agreement, verifying the high out-of-band rejection characteristics of the proposed filter.