The extended aperture method for the determination of the shadow region radiation of parabolic reflectors

In this paper the problem of the shadow region radiation of reflectors with circular apertures is examined. The approach is based on an extension of the aperture method of calculating antenna patterns. Using this extended aperture method and assuming that the reflector is an absorbing screen, the shadow region pattern within small angles from the axis of the reflector is determined. These methods are applicable only to cases where the wavelength is small with respect to the dimensions of the reflector. The main conclusions are the following: the shadow region field is primarily dependent on the illuminating field which is present on the outer edges of the reflector. For a uniform illumination the front to back ratio is of the order of
[(2\pi/\lambda)\rho]^{2}
(where
\lambda
is the wavelength and
\rho
the radius of the aperture).