Compact Wide-Stopband Bandpass Filter Based on Highly-Loaded Substrate Integrated Cavity Resonators

This paper presents a compact substrate integrated waveguide cavity-based bandpass filter with a wide stopband. The resonators are heavily capacitively-loaded to miniaturize the structures at the fundamental resonant mode. However, the heavy loading affects the resonant frequency of higher-order modes. This degrades the upper stopband rejection and narrows the spurious-free range. To mitigate the effect of closer spurious passbands, the structure of the filter is modified to incorporate stepped-impedance resonators (SIR). Compared to uniform-impedance resonators (UIR), the fields distribution in the SIRs are altered accordingly, which extends the spurious-free range of the response. To prove the concept, a compact second-order bandpass filter prototype, with capacitive loading and using stepped-impedance cavity resonators, is fabricated and measured. Measurements are shown to be in good agreement with simulations, demonstrating the validity of the proposed technique.