Analysis and design of cavity-coupled microstrip couplers and transitions

This paper describes a full-wave moment-method analysis of an aperture-coupled microstrip coupler using a resonant cavity (rectangular or circular) between the substrate ground planes. Structures of this type are useful for millimeter-wave integrated circuits and phased arrays, as they allow the use of a relatively thick central ground plane to provide isolation, as well as heat dissipation. Both four-port coupler geometries and two-port transitions are treated. Equivalent circuits are derived for each of these problems, and computed results are validated with data from the literature and with independent calculations from commercial computer-aided design (CAD) packages and measured data. These comparisons show that the solutions derived here are accurate and computationally efficient, and have the added advantage over commercial CAD packages in that equivalent-circuit parameters can be obtained directly. The vector Bessel transform is used to derive the required Green's function for the circular cavity.