Substrate-Integrated Coaxial Bandpass Filters with Symmetrical Quasi-Absorptive Response

This paper discusses a new class of substrate-integrated coaxial (SIC) resonator-based bandpass filters (BPFs) with symmetrical/two-port quasi-reflectionless behavior and quasi-elliptic-type power transmission response. The proposed quasi-absorptive SIC BPF concept is based on in-series-cascaded two-pole/one-transmission-zero (TZ) BPF modules that are resistively terminated with first-order bandstop filtering sections. High levels of filtering selectivity and low in-band insertion loss are obtained by using high-quality-factor coaxial-cavity resonators and a mixed electromagnetic (EM) inter-resonator coupling. The enabling capabilities of the engineered quasi-absorptive SIC-based BPF concept are discussed through various theoretical examples and the manufacturing and testing of an S-band prototype with the following performance metrics: center frequency of 3.75 GHz, minimum in-band insertion loss equal to 0.75 dB, 3-dB passband bandwidth (BW) of 540 MHz, and 10-dB-referred quasi-reflectionless BW equal to 1.93 GHz.