where ω ω is the angular resonant frequency of the perturbed cavity, ω ω 0 is the resonant frequency of the original cavity, E 0 and H 0 represent original electric and magnetic field respectively, μ μ and ϵ ϵ are original permeability and permittivity respectively, while Δ Δ μ μ and Δ Δ ϵ ϵ are changes in original permeability and permittivity introduced by material change.