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 Qexternal. There is no assumption of weak coupling, and the technique is ideal for the analysis of very low-Q output cavities for RF amplifiers.