Novel Compact Half-Mode SIW Bandpass Filters Based on Quad-Mode Resonator

Novel wideband half-mode substrate integrated waveguide (SIW) bandpass filters based on the quad-mode resonator are proposed in this brief. The proposed half-mode SIW bandpass filter A is composed of two half-mode SIW structures and two T-shaped slots on the top surface. The coupling matrix topology and equivalent circuit model are presented to analyze the performance. The wide band of filter A is designed on the basis of quad-mode resonator (TE101, TE102, TE103, and TE104). Besides, it can generate one transmission zero on the upper stopband due to the equivalent inductance (connecting metallic line between two half-mode SIWs) and the mutual capacitance coupling between two SIWs according to the equivalent circuit model. On the basis of filter A, two open-ended microstrip lines are added near the input and output ports (This filter is named by filter B), which will introduce two transmission zeros (TZs) in the lower stopband. The introduced TZs can improve the out-of-band rejection on the lower sideband. The resonant frequencies of the four modes are controlled by the T-shaped slots on the upper metal plate, which can reduce the overall size, significantly. The proposed two filters have compact size of 47.9mm $\mathbf {\times }\,\,14.9$ mm $\mathbf {\times }\,\,2$ mm. The prototypes have been fabricated and measured, and the measured results have a good agreement with the simulated ones.