In this letter, a compact second-order mixed coupling bandpass filter (BPF) with one controllable transmission zero (TZ) near the passband edge is presented using multilayer substrate integrated waveguide (SIW). Two arranged circular SIW resonators can be vertically coupled via the circular apertures etched on the middle metal layer while preserving a compact physical size compared with the conventional horizontally coupled filter made of the single layer. The mixed electric and magnetic coupling can be introduced by two etched circular apertures. And one controllable TZ can be created in the lower stopband for the magnetic-dominant or in the upper stopband for the electric-dominant. To demonstrate the proposed design method, a multilayer SIW BPF for WLAN application has been designed and fabricated, and the measured results show good agreement with the simulated ones.
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