Compact High-Selectivity SIW Filter Using Dual-Mode Cavity Loaded by Microstrip and Complementary Open Ring Resonators

This letter proposes a novel compact and highly selective substrate integrated waveguide (SIW) bandpass filter (BPF) with a quasi-elliptic response, integrating a microstrip resonator (MSR) and a complementary open ring resonator (CSRR) with adjustable impedance ratio in a single dual-mode SIW cavity. The working principle of the proposed hybrid structure SIW filter is analyzed in detail. The proposed filter is equivalent to a four-pole extended box-like coupling topology, which eliminates the need to consider the complex coupling signs and enables the generation of four controllable finite transmission zeros (FTZs). For the demonstration, a four-pole hybrid structure filter with a center frequency of 6.85 GHz is designed, fabricated, and measured.