Radiation from a paraboloid with an axially defocused feed

The radiation characteristics of a paraboloid with an axially defocused idealized point-source feed are derived using numerical integration of the physical-optics currents. Results from the standard "linearized" analysis are compared with results obtained without approximating various factors in the integrand. In particular, the more complete analysis reveals that the defocusing curves are not symmetrical about the focus. At values of defocusing which cause deep nulls in the defocusing curves of less-tapered feeds, the angular beam exhibits bifurcation. Consequently, these less-tapered feeds are not generally suited to beam broadening applications.