An efficient and accurate method is presented for solving the problem of radiation from a flanged parallel plate waveguide fed with an input incident TE1mode. By matching the tangential fields, an integral equation with the aperture electric field as the unknown is obtained. The method of moments is used to solve the integral equation, with the chosen basis functions term by term satisfying the £-field edge condition explicitly and the H-field edge condition implicitly. The accuracy and efficiency of the method have been assessed by a study of the convergence pattern and by comparison with other available solutions. The comparison is extended to the case in which the input mode to the parallel-plate waveguide is the TEM.