Low-Loss Directional Filters Based on Differential Band-Reject Filters With Improved Isolation Using Phase Inverter

In this letter, a novel approach to the design of directional filters (DFs) based on the differential bandstop filter and delay lines is proposed. It is shown that utilization of a phase inverter (PI) and reference line as a mode-converting phase shifter allows for obtaining theoretically ideal bandpass-to-bandstop channels isolation over an infinite bandwidth. The presented theoretical analysis is confirmed by realization of an exemplary low-loss DF with defected ground type PI covering industrial, scientific, and medical, 2.4-GHz band. The obtained measurement results show insertion loss as low as 1.1 dB and almost flat isolation response, better than 27 dB up to 6 GHz proving the validity of the presented approach.