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The beam efficiency of an antenna may be defined as the ratio of the power radiated within the main beam to the total power radiated. The beam efficiency is derived for ideal rectangular and circular apertures, as a function of the edge-to-center amplitude ratio. Random phase errors are assumed to exist across the aperture. Various types of feeds for parabolic reflectors are also considered in relation to the fraction of power which the feed directs into the paraboloid. A primary limitation on the beam efficiency of a paraboloid is shown to be produced by the surface roughness.