The technique of solving integral equations describing a microstrip antenna structure using the method of moments is well known and widely used. In the paper an efficient procedure is given to determine subsectional edge expansion functions (SEEFs) containing the correct singularities of the currents at the edges of a rectangular patch (both the current parallel to and the current normal to an edge). It is proven that the use of these SEEFs considerably improves the convergence of the calculated resonant frequency. For one configuration calculated results are compared with measured values.