High-Order Input-Reflectionless Quasi-Elliptic-Type Wideband Bandpass Filter Using Dual-Mode Slotline Resonator

A high-order input-reflectionless quasi-elliptic-type wideband bandpass filter (BPF) that exploits a complementary-diplexer-based topology is reported. Firstly, a fourth-order wideband microstrip-to-microstrip vertical transition employing a dual-mode slotline resonator as the reflective-type BPF channel is designed. It features a sharp-rejection BPF response with two close-to-passband transmission zeros (TZs). To attain broadband input-reflectionless behavior, a shunt resistively-terminated microstrip pi-shape network is used as the absorptive bandstop-filter (BSF) channel. The RF theoretical operational principle of the conceived broadband BPF is detailed. Compared to prior-art high-order input-/two-port-reflectionless wideband BPFs, the proposed BPF features not only improved passband flatness at the passband edges, but also relatively-high stopband power-attenuation levels and power-absorption ratios. For experimental-validation purposes, a two-layer fourth-order BPF microstrip prototype centered at 1 GHz is developed and tested.