Miniaturized Folded Ridged Half-Mode and Quarter-Mode Substrate Integrated Waveguides for Filter Design

This paper characterizes the performance of a miniaturized folded ridged half-mode substrate integrated waveguide (SIW) and explores its use in quarter-mode bandpass filter (BPF) design. By folding the fundamental mode of a ridged half-mode SIW (RHMSIW) around the capacitive ridge structure, an additional 50% of miniaturization can be achieved over RHMSIW and up to 87.5% compared with the standard SIW. The monomode bandwidths of both folded ridged half-mode and ridged half-mode waveguide technologies are studied, yielding 1:4.28 or greater fundamental to the second-order mode ratios for all cases and approximately twice the monomode bandwidth compared with half-mode technology. Furthermore, the study of folded ridged half-mode and quarter-mode substrate integrated cavities is presented, yielding miniaturizations compared with standard substrate integrated cavities with respect to free-space lambda equal to 96.6% and 98.0%, respectively. The design of BPFs using folded ridged quarter-mode cavity is demonstrated, with two different prototypes measured to study the choice of substrate thickness with respect to filter performance. By increasing the substrate thickness while maintaining similar cavity miniaturization, an improvement of 26.1% in unloaded quality factor between the filters is realized experimentally.