High-frequency backscattering from a metallic disc

The paper treats the backscattered field produced by a plane electromagnetic wave at oblique incidence on a perfectly conducting circular disc. The geometric theory of diffraction is used to obtain the first two terms of the asymptotic expansion at high frequencies. The first term is attributable to direct scatter from the edge of the disc and the second term arises from rays that cross the disc once. It has been found that the ray geometry is fixed for one polarisation but is aspect-dependent for the other. Owing to the axial caustic, functions must be introduced to match the wide-angle formulas to the known results for axial incidence. Bessel functions are used for the 1st-order terms and Fresnel integrals for the 2nd-order terms and evidence is presented to support this choice of function. The resulting 1st-order expression is in reasonable agreement with experimental data, and may be adequate for many purposes. However, the expressions incorporating the 2nd-order terms reveal the polarisation dependence of the scattering, and are in excellent agreement with measured data over a widt range of aspect angles.