A comparison of mode match, geometrical theory of diffraction, and Kirchhoff radiation

Using as a model a parallel-plate flanged waveguide excited by a uniform magnetic line source, the techniques of mode match, geometrical theory of diffraction, and Kirchhoff radiation are compared. In all cases the aperture fields are analyzed in terms of waveguide modes; the far-field results are Fourier transforms of the aperture fields. To within normalization, the Kirchhoff results compare surprisingly well with the two other methods except near resonance and in the low observation angle regions where the diffracted components of the field are dominant.