Aperture theory and the equivalence principle

Aperture theory in antenna courses is a topic that is usually dealt with fairly superficially at undergraduate and graduate levels. The paper takes a fresh look at aperture theory and its relationship to the equivalence principle. Some subtleties and possible misconceptions are discussed. Emphasis is placed on the mathematical analysis of the process by which the aperture is short-circuited, and the subsequent interpretation of various approximations to the aperture fields.