A Stationary Phase Method for the Computation of the Far Field of Open Cassegrain Antennas

The computation of the far field radiation patterns of large reflector antennas is of importance in predicting the performance of satellite ground stations. For example, the antenna sidelobes contribute to the system noise temperature and may cause interference with other communication systems. The open cassegrain antenna 1 is a particularly suitable configuration for obtaining low sidelobe levels, since blocking by the subreflector and its supports are eliminated. A further advantage, resulting from this feature, is that the radiation pattern can be accu431 432 T H E BELL SYSTEM TECHNICAL JOURNAL, MARCH 1970 rately predicted and it has been shown to be in good agreement with experimental results.1 The previous computations of the radiation patterns of open cassegrain antennas have been performed by precise computation of the appropriate diffraction integrals, generally requiring a double numerical integration. For large angles off-axis, such computations require considerable computation time. It is, however, for large angles that the integrals which are used for the computation of the far field radiation patterns are of a form which is suitable for approximation by the method of stationary phase. This method was initially applied to eliminate the azimuthal terms in the approximation are related to the asymptotic expansions of Bessel functions. The stationary phase approximation could therefore be modified, with the observed result that the far field radiation pattern can be computed with good accuracy also in the immediate vicinity of the main beam.