Quasi-reflectionless bandpass filters on self-absorptive resonators with flexible absorption

Reflectionless filters have been extensively studied to reduce interference in communication systems. This article presents an integral design method for quasi-reflectionless bandpass filters (BPFs) that achieve flexible responses with controllable out-of-band absorption. First, the concept of the self-absorptive resonator is proposed to facilitate the integral design. It is utilized to replace conventional resonators to produce sufficient out-of-band absorption while keeping in-band transmission. The flexible absorption features are demonstrated in two different prototypes. The narrow-band filter, based on the lumped circuit equivalence, exhibits a wide absorption band near the passband. With the transmission-line (TL) characteristics considered, the absorption circuit is modified for a high-order design to extend absorption bands. Meanwhile, the wideband filter achieves an infinite quasi-reflectionless range by employing coupled lines with open ends in between. The wideband synthesis process is applied with its TL equivalent circuit. The implementations of two prototypes on microstrip lines further verify their compact sizes and wide quasi-reflectionless ranges, as well as the versatility and simplicity of this method.