Analysis of subreflectors for dual reflector antennas

The paper presents various methods for the calculation of the field reflected from a subreflector in a dual reflector antenna system. It is demonstrated that the physical-optics (PO) solution agrees well with the geometrical theory of diffraction (GTD) for the copolar component. Significant discrepancies may appear for the crosspolar component, and it is necessary to introduce additional fringe currents in the PO solution. If the subreflector is located in the near field of the feed, special precautions must be taken. One can either subdivide the feed aperture into a number of smaller subapertures for each of which standard GTD can be applied or an alternative and more efficient method is to use complex ray analysis (CRA), where the directive feed is represented by a point source located in the complex co-ordinate space. Both methods are compared with PO solutions taking the near-field effects into account. The theoretical results are verified experimentally for a near-field illuminated offset hyperboloidal subreflector.