The efficiency of near-field Cassegrainian antennas

The aperture efficiency of a near-field Cassegrainian antenna is approximated with a Fresnel field analysis. Curves are presented which give the efficiency of the system in terms of the amplitude distribution on the feed aperture and the Raleigh distance to the subreflector. It is found that separation between feed aperture and subreflector can be as great as
\frac{1}{2}d^{2}/\lambda
without excessive loss.