The commonly used equivalent circuit based on a constant-voltage generator does not correctly characterize scattering by an aperture antenna, nor does its dual, the constant-current generator. This dilemma has been resolved by combining voltage and current sources to form a constant-power source. The power generated by this source is equated to the available power in a plane wave incident on the aperture. The voltage and current of the two sources and determined from a knowledge of the aperture area A//0, the impedance Z//A, the aperture efficiency eta //0 and the relative power pattern p( theta , phi ) of the antenna.
