<i>X</i>‐Band Air‐Filled Coaxial Bandpass Filter With Wideband Input‐Reflectionless Performance

ABSTRACT Reflectionless bandpass filters (BPF) have been realized in the waveguide and planar circuits; however, the literature does not report reflectionless air‐filled coaxial BPF. In this article, an X ‐band input‐reflectionless air‐filled coaxial BPF is presented. The proposed filter has two transmission zeros (TZs) at the lower and upper stopband, respectively, to improve the selectivity. An equivalent circuit of this filter is provided to explain its reflectionless filtering response, which contains a BPF section and a bandstop filter (BSF) section. For verification, a reflectionless BPF centered at 10.2 GHz with an in‐band fractional bandwidth (FBW) of 6.39% has been designed and fabricated. The frequency range of measured 10‐dB return loss is from 4.75 to 13.00 GHz (10‐dB‐absorption FBW of 80.9%), showing a very attractive wideband reflectionless filtering behavior.