An analysis is presented which employs the uniform geometrical theory of diffraction (UGTD) for determining the near fields diffracted by a paraboloid either with or without a conical flange attached to its circular rim when an axially propagated plane wave is incident on the concave or convex portion of the paraboloidal reflector. The field correlation theorem is employed to determine the power coupled by a prime focus feed associated with the paraboloid which is being iliuminated by a uniform plane wave. This analysis enables one to avoid approximating the prime-focus feed (PFF) as a point source radiating spherical waves. Based on this analysis, the front-to-back (F/B) ratio of unflanged and flanged paraboloids is computed. Computed results show satisfactory agreement with the available measured as well as computed results based on alternative procedures. Further, the variation in the on-axis gain of a prime-focus reflector (PFR), when the feed is displaced from the focus is also studied. Typical computed results of this study are presented which are compared with the available measured data. Computed results on F/B ratio of paraboloids (flanged or unflanged) illuminated by a PFF whose radiated field exhibits phase variation over a constant radius are also presented. © 1991 IEEE