Compact and Wide Stopband Substrate Integrated Waveguide Bandpass Filter Using Mixed Quarter- And One-Eighth Modes Cavities

This letter presents the design of mixed quarter- and one-eighth modes substrate integrated waveguide (SIW) bandpass filter (BPF). The transmission zeros (TZs) of the proposed SIW BPF can provide wide stopband response and high selectivity characteristics by using different mode SIW cavities. For validation, two- and three-stage SIW BPFs with Chebyshev response were designed at a center frequency ( f_{0} ) of 8 GHz. The measured results are consistent with the simulations. For two-stage BPF, the insertion loss smaller than 0.9 dB is measured within the passband of 0.65 GHz (7.75-8.4 GHz). The return loss higher than 19.7 dB is measured within the same passband. The spurious is produced at around 18 GHz ( < 2.25f_{0} ). The stopbands are attenuated more than 17.31 dB from the dc to 5.68 GHz ( 0.71f_{0} ) and from 9.28 GHz ( 1.16f_{0} ) to 16.67 GHz ( 2.08f_{0} ). The TZs are produced at 10 GHz and around 18 GHz due to a small cross-coupling between source/load and the interaction of the higher resonant modes of cavities, respectively. For three-stage BPF, the \vert S_{21}\vert and \vert S_{11}\vert smaller than -1.3 dB and -18 dB are measured within the passband of 7.57-8.45 GHz fractional bandwidth (FBW = 11%), respectively. The TZs are produced at 1.102, 1.9, and 2.39f_{0} and provide higher selectivity and attenuation compared to two-stage BPF.