A New Folded Coupling Reflectionless Bandpass Filter with Broadband Ultra-Low Reflection Property and Very High Frequency Selectivity

This paper proposes a new generalized 2N-resonator coupled reflectionless bandpass filter (RBPF) that can suppress reflection less than −60 dB for all frequencies as well as high skirt selectivity using transmission zeros (TZs). Conventional RBPFs are composed of bandpass sections combined with hybrids or bandstop sections. However, such configurations have a serious problem that a minimum return loss level is about 10–15 dB in a limited bandwidth. In this paper, we design and test two microstrip RBPFs with the proposed folded coupling configuration. The design examples are 2-GHz band four- and eight-resonator coupled RBPFs, showing a prescribed passband, a return loss level greater than 20 dB from dc to spurious resonant frequency, and a very large out-of-band rejection.