Formula for the Field Excited in a Rectangular Cavity With a Small Aperture

A formula is derived for the field excited in a perfectly conducting rectangular cavity by an external field incident on a small aperture or localized group of apertures in a cavity wall. It is assumed that the cavity is part of a larger system and that a separate estimate has been made of the external field near the aperture. The formula is shown to produce results in good agreement with results derived using specialized codes.