Synthesis and design of a suspended substrate capacitive gap-parallel coupled line bandpass filter with one transmission zero

A design method for a suspended substrate stripline bandpass filter with one transmission zero at a given frequency is presented. We use a generalized Chebychev lowpass prototype based on admittance inverters and parallel resonators. The filter topology is called "Capacitive gap-parallel coupled line filter" because the synthesis is based on the equivalent circuits of parallel-coupled lines and end-coupled (gap) lines. A CAD software has been developed for the synthesis of such suspended substrate filters and a prototype filter at 8.5 GHz is given as an example.