All-Frequency-Two-Port-Quasi-Absorptive Planar Bandpass Filter With Quasi-Flat Group Delay and Multi-Transmission-Zero Extended Stopbands

A microwave planar bandpass filter (BPF) with in-band-quasi-flat-group-delay characteristics and two-port-quasi-reflectionless behavior for an infinite spectral range is presented. It is composed of the in-series cascade connection of three multipath-delay-line transversal filtering sections (TFSs) with Wilkinson-type power-division/combination stages. In this manner, by means of the composite action of these TFSs, an in-band-quasi-Bessel-type bandpass filtering response with multi-transmission-zero (TZ) stopbands and two-port-quasi-absorptive nature for a finite frequency interval is attained. To further extend such quasi-reflectionless bandwidth from dc to infinite theoretically, as well as the stopband range, one-port-quasi-absorptive wide-band filtering cells are inserted at both the input and output accesses of the overall quasi-flat-group-delay BPF. For experimental demonstration purposes, a microstrip prototype centered at 5 GHz is designed, manufactured, and characterized.