A quasi-elliptic response triple-mode SIW bandpass filter with controllable transmission zeros

This letter reports a novel quasi-elliptic bandpass filter (BPF) using a new triple-mode circle-shaped substrate integrated waveguide (SIW) cavity. The triple-mode SIW cavity is achieved by a dual-mode circle-shaped SIW cavity with a floating capacitive-loaded metallic patch. The resonance of one higher-order mode is independently shifted to that of dual modes to form a three-pole passband. Three finite transmission zeros (FTZs) can be produced, and two of them can be controlled well by the angle between two feeding lines. For the demonstration, a prototype filter was designed, fabricated and measured. Good agreement between measured results and simulated results is observed. The proposed triple-mode SIW filter has the merits of compact size, high selectivity and controllable bandwidth as well as FTZs.