The small aperture coupling theory is improved by introducing the radiation reaction fields as part of the polarizing field acting on the aperture. The principles of power conservation and reciprocity are used to determine the dipole strengths for radiation on either side of the aperture with the aperture closed. The theory thus enables coupling between dis-similar regions to be treated in a straightforward manner. The theory leads to physically meaningful equivalent circuits for describing small aperture coupling. Cavity coupling and radiation into free space are treated as examples.