For rectangular waveguide cavity, field distribution of dominant T E 10 n mode is well known. Ideally, the material to be measured is introduced into the cavity at the position of maximum electric or magnetic field. When the material is introduced at the position of maximum electric field, then the contribution of magnetic field to perturbed frequency shift is very small and can be ignored. In this case, we can use perturbation theory to derive expressions for real and imaginary components of complex material permittivity ϵ ϵ r = ϵ ϵ r ′ + j ϵ ϵ r ″ as: