Design of Quarter-Mode SIW BPF with Ultra-Wide Stopband and Impedance Matching Functionality

This paper demonstrates the design of quartermode (QM) substrate integrated waveguide (SIW) bandpass filter (BPF) with ultra-wide stopband and impedance matching functionality. Ultra-wide stopband characteristics can be obtained by embedding dumbbell shape slots (DSS) on the ground metal plane at the input and output ends of the proposed BPF to suppress spurious response of higher modes. The matching functionality can be achieved by detuning the resonance frequencies of the first or last QM SIW resonators due to the reactive components of the complex termination impedances. For concept validation, a two-stage QM SIW BPF is designed and fabricated with the center frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(f_{0}\right)$</tex> of 10 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{G H z}$</tex> and the termination impedances of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(20+j 15) \Omega$</tex>-to-50 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Omega$</tex>. The minimum attenuation level of 26.7 dB is measured from DC to 7.2 GHz at the lower stopband and 12.85 GHz to 40 GHz at the higher stopband. Since the proposed SIW BPF is realized using QM SIW cavity, the circuit size is much smaller compared to the SIW BPF realized using full-mode SIW cavity.