A cutoff-waveguide bandpass filter using antiresonant elements for sharp passband skirt and wide stopband characteristics

A dual-behavior resonator used for waveguide and microstrip filters has an advantage that multiple transmission zeros can be generated by its antiresonances, but has a problem that bandwidth of upper stopband is narrow owing to its spurious resonances. To suppress such undesired resonances, a compact cutoff-waveguide bandpass filter introducing a new resonance mechanism is proposed. The filter is simply constructed by successive arrangement of planar antiresonant elements in cutoff waveguides. Both the resonance and the antiresonance are produced by the antiresonant elements with the help of intrinsic inductive elements of the cutoff waveguide. As an example, a bandpass filter having high skirt selectivity, low insertion loss at the passband, a wide stopband response, and large attenuation is designed at X band. The effectiveness of the filter is verified by numerical and experimental demonstrations.