Electromagnetic field in the caustic region of a wide angle spherical antenna under oblique parallel beam illumination

Using the method of the angular spectrum of plane waves, we have derived a formulation that can be used to determine the electromagnetic field distribution in the focal region of a large-aperture spherical antenna receiving an obliquely incident and linearly polarized plane monochromatic wave. We have used our formulas to determine the three-dimensional field distribution in the caustic region for selected obliquities of the incident beam and for two incident polarizations. Knowledge of this field distribution is essential for the design of an optimized feed structure for the antenna. We have presented and discussed some numerical results concerning the TM and TE waves along the oblique axis of incidence, calculated for two operating frequencies of the giant Arecibo antenna.