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Characteristics of a feed that utilizes a unique method to produce a conically scanning main beam are discussed. Model and full scale test data are included to make the feed operation more clearly understood. Certain criteria such as gain, side lobe level, and polarization must be considered when designing a primary feed for a paraboloid reflector. Also, the automatic tracking requirement calls for some means of producing an error signal for given off-target angles. The feed discussed is a circular waveguide excited by orthogonal probes fed 90Â° out of phase to produce circular polarization. The electrical phase center of the feed is shifted around the focal point of the 60-foot diameter paraboloid reflector by rotating a hemispherical tapered-dielectric prism at the aperture of the waveguide. A ground plane around the waveguide mouth controls the illumination taper across the paraboloid aperture to yield equal E and H plane secondary beamwidths.