Numerical modeling of cavities with low external Q

We describe a new simulation technique to find the resonant frequencies and Qs of cavity eigenmodes when the cavity is coupled to an external waveguide. The method treats the cavity first as a closed system to find a set of lossless eigenmodes, then couples this system to a representation of the external waveguide to determine the coupled mode frequency and Q
_external
. There is no assumption of weak coupling, and the technique is ideal for the analysis of very low-Q output cavities for RF amplifiers.