Approximation Theory on the Electromagnetic Fields in Metal Cavity

A new method in perturbation theory of electromagnetic fields in metal cavity has been proposed in this paper. When a medium is introduced in metal cavity, the electromagnetic fields and oscillating frequency will be changed. The changed fields can be expanded as the fields of all modes in the original empty metal cavity. The expressions of the fields and the frequency in the various orders of approximation have been derived, that is important for such applications as the precise measurement of high permittivity of microwave materials.