A Miniaturized, Low‐Loss, and Self‐Shielded SIW Bandpass Filter With Three Transmission Zeros

ABSTRACT This letter presents a miniaturized, low‐loss, and self‐shielded substrate integrated waveguide (SIW) bandpass filter (BPF) based on a mixed‐mode cavity (MMC). By employing two TE 1/2,0,1 patch modes and one TE 101 cavity mode as resonant modes, and two TEM 1/2,0,1 patch modes as nonresonant modes, a third‐order filtering response with three transmission zeros (TZs) is achieved within a single cavity. The high unloaded quality factors ( Q U ) of the resonant modes ensure low insertion loss. A prototype centered at 13.87 GHz with a 3‐dB fractional bandwidth of 4.2% is fabricated and measured. The measured results show a minimum insertion loss of 1.47 dB, a compact footprint of only 0.5 λ d 2 , and three TZs at 11.82, 14.31, and 14.78 GHz. Compared with other state‐of‐the‐art self‐shielded SIW BPFs, the proposed filter offers a favorable trade‐off among size, loss, and selectivity.