Capacitive End-Couplings in Combline Microwave Cavity Filters with Probe Parallel to Resonators' Axes: Comparison and Design Guidelines

End coupling plays an important role in cavity filters since it realizes the desired external quality factors for end resonators. Despite this, few designs have been introduced and analyzed in the past and this is a limitation in today’s state-of-the-art cavity filters. The work then aims at widening the available selection for end coupling designs by introducing three different capacitive couplings with probe parallel to resonators’ axes. The parameters characterizing the three designs are varied to provide design curves for the external quality factor and design guidelines for the proposed end couplings are illustrated considering a filter for the El Galileo band as example. Results show that for the analyzed end coupling designs the external quality factor ranges from tens to hundreds. The proposed capacitive end coupling designs find application in CAD tools.