Input-Absorptive Quasi-Elliptic-Type Cavity Bandpass Filter Design

A 2
^nd
-order quasi-elliptic-type input-absorptive filter with two transmission zeros (TZs) at close-to-passband positions is proposed in this paper based on the diplexer model, and the extracted pole resonators. The equivalent circuit of this design includes two channels, each divided into two sub-networks composed of bandpass resonators (BPRs) and bandstop resonators (BSRs). The bandstop channel consists of two extracted pole BSRs reflecting the in-band power. The bandpass channel has two cascaded BPRs, including a single-mode resonator and a dual-behavior resonator responsible for introducing the TZs and reflection zeros (RZs). TZs and RZs generated by the BPRs and BSRs are investigated mathematically in this paper. Finally, a 2
^nd
-order metallic quasi-elliptic-type input-absorptive cavity bandpass filter based on WR-28 waveguide was designed, simulated, fabricated, and measured for practical validation. A good agreement was obtained between the theory, simulated, and measurement results.