Power reciprocity in antenna theory

An attempt is made to obtain a reciprocity relation in which certain quantities related to the time-averaged electromagnetic power flows in the transmitting and in the receiving situation occur for a single antenna. It is shown that such a relation can be obtained provided that, in the transmitting situation, the antenna system is fed at a single accessible port. In the receiving situation, a load is connected to this port. The most general case (under the restriction of a single accessible port) is discussed, and the proof is a rigorous one, based throughout on electromagnetic field concepts and Lorentz's reciprocity theorem for electromagnetic fields.