A Compact Dual-Mode Multilayer Substrate Integrated Waveguide Quasi-Elliptic Bandpass Filter

A compact dual-mode substrate integrated waveguide quasi-elliptic bandpass filter (BPF) based on the multilayer printed circuit board process is presented in this paper. The proposed BPF introduces two pairs of rectangular coupling slots on the first middle metal layer to adjust the coupling strength between the TE
101
mode in the upper resonators and high-order TE
102
, TE
201
and TE
202
modes in the lower dual-mode resonator. Meanwhile, the vertical couplings can wide the passband and improve the stopband performance. The BPF also realizes the topology of two cascaded trisection structures to generate transmission zeros (TZs) by utilizing the orthogonal high-order TE
102
and TE
201
modes. Measured results show that the BPF has a wide 3-dB bandwidth of 3.54 GHz (13.6%) at central frequency around 26 GHz, and the minimum insertion loss in the passband is 1.66 dB. The BPF realizes the high selectivity with four TZs and has a compact size of about 0.71 × 0.65 ${\lambda _0}^2$, where λ
0
is the wavelength (in free space) of the center frequency.