In antenna terms, solar power collectors arguably reside largely in the 'aperture antenna' camp. Consequently, Chap. 4 endeavours to provide the reader with a fundamental appreciation of the radiation characteristics of aperture antennas by deriving the key integral equation for the magnetic vector potential Am from the Maxwell equations. The Am equation, as we shall see in Chap. 8, is the route into geometrical optics, an analytical technique which represents the 'bedrock' of reflector antenna design. With a knowledge of Am that has been secured, the radiation properties of some regularly shaped apertures are then examined, and this leads to the derivation of spectrum functions which relate the far radiation field of the antenna, to the near or aperture field, through a Fourier transform pair. The relationships that exist between antenna directivity, gain, radiation efficiency and sidelobes and how these basic properties are influenced by the aperture field distribution of the given antenna are studied in detail, for both two-dimensional and three-dimensional radiation pattern embodiments.