Limited Electronic Scanning with a Near-Field Cassegrainian System

The scanning characteristics of a near-field Cassegrainian antenna excited with a relatively small planar array are investigated theoretically and experimentally. The analysis uses both geometrical optics and scalar diffraction theory. The technique is a viable approach to the problem of modifying an existing high-resolution dish radar for limited scan capability. Feed requirements can be satisfied by virtually any planar array concept, i.e., phase scanning, frequency scanning, multiple-beam-forming networks, etc. The scan range is roughly approximated by a fixed angular field of view; hence, the number of beamwidths scanned with electrically large apertures can be very high. Further, the system is free from the coma lobes usually associated with off-axis beams from a paraboloid. Design curves are presented from which one can determine the secondary characteristics - gain, scan loss, sidelobes, etc. - for a given array/reflector configuration.