Aperture fields and gain of open-ended parallel-plate waveguides

The fields at the open end of a parallel-plate waveguide with a mode incident are calculated. For plate spacings greater than about 0.6 wavelengths, these are well approximated by incident and first-order diffracted fields. Consequently single diffraction at the aperture alone is often adequate for pattern calculations and, although no more complicated, is superior to the Kirchhoff result, even for on-axis calculations. Numerical values for Kirchhoff, geometrical theory of diffraction and exact on-axis gain demonstrate this. In contrast, for TEM incidence, both methods yield the exact on-axis gain. Application of these results to the improvement of gain prediction of horns is discussed.