Design techniques for microwave bandpass filters using re-entrant resonators are presented. The key feature is that each re-entrant resonator in the filter generates a passband resonance and a finite frequency transmission zero, above the passband. Thus an Nth degree filter can have N finite frequency transmission zeros with a simple physical realization. A physically symmetrical 5 pole re-entrant bandpass filter prototype with 5 transmission zeros above the passband was designed and fabricated. Measured results showed good correspondence with theories.
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