Distributed-Element Reflectionless Bandstop Filter With a Broadband Impedance Matching

In this letter, we present a systematic design method for a reflectionless bandstop filter using distributed-element resonators. One unique feature of the presented filter structure is that it is capable of producing a broadband impedance matching at both ports with a flat passband. For verifying the design approach, a second-order Butterworth reflectionless bandstop filter centered at 2 GHz with fractional bandwidth of 8% has been designed, fabricated, and measured. The measurement shows that the return losses (S11and S22) are overall larger than 10 dB from DC to 3f0.