ANALYSIS AND MEASUREMENT OF ELECTROMAGNETIC SCATTERING BY PYRAMIDAL AND WEDGE ABSORBERS (DIFFRACTION, UTD, GTD)

By modifying the reflection coefficients in the Uniform Geometrical Theory of Diffraction coefficients a solution that approximates the scattering from a dielectric wedge is found. This solution agrees closely with the exact solution of Rawlins which is only valid for a few minor cases. This modification is then applied to the corner diffraction coefficient and combined with an equivalent current and Geometrical Optics solutions to model scattering from pyramid and wedge absorbers.;Measured results from 12" pyramid absorber from 2 to 18 GHz are compared to calculations assuming the returns add incoherently and assuming the results add coherently. The measured results tend to be between the two curves. The pyramid calculated results are also compared to bistatic measurements. In the nose-on regions, all the terms are needed while, near grazing incidence, the tip diffraction dominates the return. Measured results from 8" wedge absorber is also compared to calculations with the return being dominated by the wedge diffraction.;The procedures for measuring and specifying absorber performance are discussed and calibration equations are derived to calculate a reflection coefficient or a reflectivity using a reference sphere. The absorbers studied tend to act highly dispersive, so specifying a reflectivity is preferred to specifying a reflection coefficient.;Shaping changes to the present absorber designs are introduced to improve performance based on both high and low frequency analysis. The change involves making the tips more pointed, curing the sides of the pyramid to break up the reflected fields, and making the base angle smaller. These changes have an overall effect of smoothing the transition between free space and the absorber material. Some prototypes were built and tested.