Design of a Dual-Mode SIW Bandpass Filter With Multi-Transmission Zeros

In the paper, a method for cascading two dual-mode substrate integrated waveguide (SIW) filters by loading slots is proposed, resulting in two pairs of transmission zeros (TZs) observable on both sides of the passband. A corner of the single-mode SIW cavity is folded inward, and a perturbation via is placed diagonally to achieve the conversion between single-mode and dual-mode operation. The dual-mode SIW filter primarily operates in the TE102 and TE201 modes. By adjusting the width of the coupling window and the length of the two slots, the resonance points within the passband can be tuned. The positions of the out-of-band TZs can be modified by changing the offset distance between the input/output ports and the centerline, as well as the distance between the slots. The two dual-mode SIW cavities achieve magnetic coupling through a coupling window structure, while electric coupling is realized by loading slot structures between the coupling windows. The combined effects of magnetic and electric coupling create two pairs of TZs on both sides of the passband, enhancing frequency selectivity. A fourth-order SIW filter with two pairs of TZs was simulated, fabricated, and measured. The measured frequency response closely matches the simulated results, with the center frequency $f_{0}$ of 29.3 GHz, the insertion loss of 0.81 dB, and the return loss better than 16.8 dB. Four TZs were observed at 23.05, 27.11, 31.46, and 37.74 GHz, with suppression better than 25 dB. This filter is applicable to microwave and radio-frequency circuits and systems.