Exact Image Method for Field Calculation in the Substrate of the Microstrip

The well-known technique of computing the electromagnetics fields above the substrate of a microstrip antenna is extended for the field calculation in the substrate medium. These fields are needed especially in determing the input impedance of a patch antenna fed through the ground plane. One image is situated above the substrate accounting for the air-substrate interface and the other image is situated below the substrate accounting for the ground plane-substrate interface. The image functions are computed from an infinite set of poles and a branch cut integration by using the Mellin inverse formula.