Wide-Angle Radiation Due to Rough Phase Fronts

A rough or nonuniform phase front, be it acoustical, radio, or optical, usually degrades the desired performance of components which transmit, reflect, or receive the wave. The effect is well known in the field of microwave antennas, where lenses are required to have sufficient homogeneity of dielectric constant and reflectors sufficient smoothness of surface to produce uniform phase fronts. Likewise, the quality of optical components is specified, among other things, by ability to reproduce or modify wavefronts in a prescribed manner without undue distortion. If roughness is introduced into a wavefront by a component, some of the power is no longer radiated in the desired specular direction; it propagates at angles well removed from that direction. This effect can be described by a system of modes in the radiating aperture, each of which radiates in a specified direction.* In practice, it is difficult to describe the roughness properly. Consider, for example, the reflector of a microwave antenna. If large, seldom is it constructed from a single sheet of metal. More often, sheets are cut and shaped to form modules of given dimension, these then being assembled with the desired precision to construct the antenna. One might expect, therefore, that the power spectrum of the wavefront would have a com* O f t e n called t h e a n g u l a r power s p e c t r u m . 2285