An extension of the multiport network modelling technique applied to rectangular microstrip patches on thin substrates covered with a dielectric layer is reported. The edge admittance networks for the radiating edges and the mutual coupling network for the external interaction between the two radiating edges are obtained from the spectral domain solution for fields of a magnetic current element placed on a conducting ground and covered with a dielectric layer. Results of the analysis for single-port and two-port patches are compared with experimental results. The good agreement validates the model and the analysis procedure proposed.