On-axis defocus characteristics of the paraboloidal reflector

Lack of an adequate test site often requires the use of on-axis defocusing to obtain simulated Fraunhofer radiation patterns in the optical-Fresnel region from microwave reflector-type antennas. This note presents a way of determining the required amount of defocus from the viewpoint of aperture phase correction. It is shown that the important part of the main lobe of the radiation pattern can be approximately reproduced in the optical-Fresnel region with proper amount of defocus. For a given amplitude illumination function, the change in beamwidth with the position of the primary source can also be predicted.