Reflection from the aperture of a long -plane sectoral horn

The complex reflection coefficient of a long -plane sectoral horn is derived using the geometrical theory of diffraction. Single diffraction alone is shown to be sufficient to describe the contribution from the (widely separated) edges, while singly and doubly diffracted fields from the edges are included by using an open waveguide result derived elsewhere. A relatively uncomplicated expression is obtained for the complex reflection coefficient of the horn which agrees well with experiment for horns with slant wall lengths greater than about guide wavelengths.