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When two complementary sources are combined in the proper amplitude and phase, desirable radiation characteristics for feeding a circular aperture are obtained. It is shown that when the feed is achieved there results a circular beam cross section which optimizes the efficiency of illumination of a circular aperture. The back radiation from the feed is down 30 db from that in the forward direction, minimizing interference effects between feed and aperture. It is the purpose of this thesis to show how a feed composed of complementary sources has been physically realized and to present and discuss experimental radiation and impedance data. It is well known that the radiation pattern of an electric dipole is a circle in the plane and a figure 8 in the plane. An open-ended coaxial line carrying the mode is similar to a magnetic dipole; i.e., the plane is nearly circular while the plane is like a figure 8. These two sources have been combined to produce a feed whose - and -plane patterns are of equal width. The complementary source idea has been applied to feeds of both linear and circular polarization. The linearly polarized feed is excited from rectangular waveguide and is simple to fabricate. It can be easily matched over a broadband. This feed has been used to illuminate a 20-inch parabola with the result that the secondary and planes are of equal width and the side lobes are 30 db down from the main radiation. The circularly polarized feed is excited from a circularly polarized mode in coaxial line. The radiating structure maintains circular symmetry and the axial ratio remains essentially constant over a large portion of the beam.