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Nonplanar log-periodic-type antennas are attractive as wideband reflector feeds since they can be arranged to provide an axially symmetric radiation pattern that is stable over a wide bandwidth with near constant input impedance. However, these antennas have a frequency-dependent phase center that causes de focusing and degrades reflector antenna performance when used in wideband systems. This letter presents a technique, which includes radiation pattern as well as defocusing effects, to predict an upper bound for reflector system performance when a nonplanar log-periodic feed is used, resulting in useful design guidelines. Predicted results compare favorably with physical optics simulations.