Aperture Coupling Between Microstrip and Resonant Cavities

This paper presents a simple analysis for the coupling between microstrip and a cavity through an aperture located in the substrate ground plane. The analysis is based on Wheeler's equivalent-energy concept for small-hole coupling and an approximate parallel-plate waveguide model for the microstrip. The theory appears adequate for most design purposes, and has been used successfully in the design of stabilizing cavities for experimental 12-GHz low-noise FET oscillators.