Microwave Bandpass Filters Using Re-Entrant Resonators

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 N th degree filter can have N finite frequency transmission zeros with a simple physical realization. A physically symmetrical five-pole re-entrant bandpass filter prototype with five transmission zeros above the passband was designed and fabricated. Measured results showed good correspondence with theories.