Practical Design Method of Distributed-Element Reflectionless Bandstop Filters

A practical reflectionless bandstop filter topology is presented in this paper. One unique feature of the presented distributed-element reflectionless filter circuit compared to others is that it only requires to design and tune N+1 pairs of coupled lines for an Nth-order filter. Closed-form design equations for a second-order filter have been provided, and they can be expanded for higher-order filters. For verifying the presented filter topology and the design method, a reflectionless bandstop filter has been designed, fabricated, and measured. The measurement shows that the fabricated filter has the return loss larger than 20 dB in the frequency range of interest, with having the predefined transmission response.