Novel Wideband Microstrip Absorptive Bandstop Filter Utilizing Coupled-Line Architecture

Reflectionless and high-attenuation characteristics are crucial for protecting sensitive front-end components, such as power amplifiers and mixers, in modern communication systems. This paper proposes a novel microstrip absorptive bandstop filter (ABSF) that offers comprehensive protection across a wide frequency range. The filter achieves an exceptionally wide stopband bandwidth through a core topology utilizing two coupled lines interconnected by central transmission sections. Crucially, high absorption is realized by incorporating resistor-terminated absorptive transmission lines at the input port. Complete design equations and the theoretical framework necessary for synthesis are detailed. The fabricated prototype demonstrates outstanding measured performance, achieving a stopband attenuation of 40 dB and a minimum absorption of 18 dB over a remarkable fractional bandwidth of 62.9%. This rigorously validates the proposed absorptive bandstop filter as a highly practical and effective solution for wideband component protection.