Case study of phase-center relationships in an asymmetric cassegrainian feed system

Determination of the phase center of the field scattered from the tilted hyperboloidal subreflector in an asymmetric Cassegrainian antenna system is carried out using several techniques. The caustic curves associated with the virtual reflected rays in the asymmetric system are determined from a differential-geometry analysis of the problem. A geometrical-optics analysis is carried out to determine a best-fit phase-center position by a least-squares curve fitting technique. The phase of the scattered field, as determined by a diffraction-theory analysis, is also used in a best-fit phase-center determination. The phase centers of the scattered field for six hyperboloid diameter-to-wavelength ratios (from 59.54 down to 2) and the point of intersection of the two caustic curves are all found to be within approximately 1/10 wavelength of each other (for each characteristic wavelength). A substantial decrease in rms phase variation around the best-fit phase centers is seen, compared to the phase variation around the paraboloid prime focus.