  A new method based on parameters iteration technique has been developed to
determine the optimal subreflector position for shaped Cassegrain antennas to
improve the electromagnetic (EM) performance distorted by gravity. Both the
features of shaped surface and the relationship between optical path difference
(OPD) and far field beam pattern are employed. By describing the shaped
dual-reflector surface as a standard discrete parabola set, we can utilize the
optical features of the standard Cassegrain system in the classical OPD
relationship. Then, the actual far field beam pattern is expressed as the
synthesis of ideal beam and error beam by decomposing subreflector adjustment
parameters using mechanical-electromagnetic-field-coupling-model (MEFCM).
Furthermore, a numerical method for determining optimal subreflector position
is presented. The proposed method is based on the iteration technique of
subreflector adjustment parameters, and the optimal far field pattern is used
as the iteration goal. The numerical solution of optimal adjustment parameters
can be obtained rapidly. Results of a 25 m Shaped Cassegrain antenna
demonstrate that the adjustment of the subreflector to the optimal position
determined by the proposed method can improve the EM performance effectively.
