Abstract
Application of the incident/reflected wave boundary condition to the wave equation for a weakly irregular waveguide leads to a spectral domain analysis of the open cavity with a typical maximum-field spectrum shown. It is shown by a spectral model that the cold cavity field profile of a given mode depends on the method and frequency of its excitation. Coupling to an unmatched external circuit is shown theoretically and experimentally to strongly modify the properties of the open cavity. Ohmic loss on the wall has been incorporated into the formalism by the addition of a loss factor to the wave equation.