Spatial power combining of many MMIC amplifiers at millimeter wave frequencies using a fixed array of microstrip antenna elements places unique demands on dielectric media. The substrate must be relatively thick to allow space for MMIC placement, must provide rather high thermal conductivity to disipate MMIC heat, and be of high dielectric constant to shrink circuit element dimensions. Presently, microstrip antenna models require a low dielectric constant substrate to be valid. This paper presents a modified transmission line model on the model of Pues and Van de Capelle which addresses the problems of thick, high microstrip antenna elements. The goal of the model was to guide design of a microstrip array antenna suitable for a spatial power combined module.