Design of a three-dimensional gap-coupled suspended substrate stripline bandpass filter

A three-dimensional gap-coupled suspended substrate stripline (SSS) bandpass filter, based on the modified configuration of the classic end-coupled filter, is presented. It consists of printed half-wavelength resonators placed on the same and/or the opposite sides of the suspended substrate. These resonators orient themselves in arbitrary directions. The filter is constructed in a bent channelised housing that suppresses any possibility of exciting higher-order modes, except the dominant quasi-TEM mode of propagation. The de-embedded discontinuity parameters are validated by comparing them with those obtained by experiment and full-wave approaches for the microstrip open-end and the SSS end-coupled problems, respectively. The measured results for the experimental 21-22 GHz bandpass filter agree very well with the theoretical predictions considering the conductor losses (5.57×10^-3 dB/mm) of the SSS. It has less than 1.4 dB insertion loss and greater than 10 dB return loss in the passband