Design of axially-symmetric Cassegrain and Gregorian configurations with reduced spillovers

One of the tasks faced by antenna designers is the accomplishment of radiation patterns satisfying restricted specifications. Generally, low sidelobe levels are required in conjunction with low spillover losses. In a dual-reflector configuration, these losses are mainly related to the feed radiation past the subreflector (forward spillover) and the subreflector radiation past the main reflector (backward spillover). The forward spillover may be reduced by a convenient metallic shield placed around the subreflector rim. However, the metallic shield can be understood as an extension of the subreflector, giving rise to the possibility of shaping the whole structure (subreflector/shield) as a single one with a relatively larger feed taper, simultaneously obtaining reduced forward and backward spillovers. The article describes a simple formulation, based on the uniform theory of diffraction (UTD), relating the subreflector-edge-directed feed illumination (desirably low enough to reduce the forward spillover) with the associated aperture-rim electric field (prescribed for the antenna shaping), while naturally providing a large subreflector radiation roll-off.