A lossy metal-wall cavity resonator that extends well beyond perturbation theory limits
is studied. An exact analytical solution is employed for the spherical cavity resonator, having walls
transformed from being a perfect electrical conductor (PEC) to free space. This model then acts as an
ideal benchmark reference standard. A plane-wave approximation is then derived. Independent full-wave
numerical modeling of the spherical cavity resonator is undertaken using eigenmode solvers within two
well-known commercial, industry-standard, simulation software packages (HFSSTMand COMSOL). It
has been found that the plane-wave approximation model accurately characterizes the results generated
by these solvers when equivalent ¯nite conductivity boundary (FCB) and layered impedance boundary
(LIB) conditions are used. However, the impedance boundary (IB) condition is accurately characterized
by the exact model, but the precise value of complex wave impedance at the wall boundary for the specific
resonance mode must first be known a priori. Our stress-testing results have profound implications on
the usefulness of these commercial solvers for accurately predicting eigenfrequencies of lossy arbitrary
3D structures. For completeness, an exact series RLC equivalent circuit model is given specifcally
for a spherical cavity resonator having arbitrary wall losses, resulting in the derivation of an extended
perturbation model.