Single/Multi-Band Coupled-Multi-Line Filtering Section and Its Application to RF Diplexers, Bandpass/Bandstop Filters, and Filtering Couplers

A type of compact single/multi-band coupled-multi-line filtering section and its application to the design of RF filtering devices are presented. This one-port filtering cell is made up of N+1 intercoupled quarter-wavelength transmission-line segments for the generation of a transfer function with a total of N+1 transmission zeros (TZs) distributed at both sides of all its N first-order passbands when arranged in the transmission mode. Design formulas for the produced poles and TZs of its corresponding normalized coupling-routing diagram and illustrative theoretical responses are provided. Furthermore, for experimental demonstration purposes, microstrip prototypes of the following RF filtering components that exploit the conceived filtering section are manufactured and measured: 1) a quasi-elliptic-type diplexer with a dual-band bandpass filter (BPF) junction; 2) two input-reflectionless triple-band BPF and bandstop filter (BSF) circuits; and 3) an out-of-phase equal-power-division bandpass filtering coupler with input-quasi-reflectionless behavior.