An extended doublet bandpass filter implemented with microstrip resonator and substrate integrated waveguide cavity

An extended doublet bandpass filter with compact size and good spurious suppression characteristic is proposed, where a dual-mode microstrip resonator and a single-mode substrate integrated waveguide (SIW) cavity are utilized. The relationship between the location of its transmission zeros and the impedance ratio of the microstrip resonator is analyzed theoretically. Our proposed SIW filter only occupies an area of 0.69λ₀²/ε_r and has a wide upper stopband under 15 dB. The predicted performances are demonstrated by the reasonable agreement obtained between the simulated and measured S-parameters.