# Measurement of the dielectric constant and loss tangent of materials in microwave frequency band using a rectangular waveguide cavity

__Theory Continued...__

__Cavity Perturbation Technique__

As we know, that a piece of sample material affects the resonant frequency (f_{r}) and quality fator (Q) of the cavity. From these parameters, the complex permittivity (ε_{r}) or permeability (µ_{r}) of material can be calculated at a single frequency. There are many different types of cavities and methods. For calculation of these parameters, most widely used method is Cavity Perturbation Technique. This method uses a rectangular waveguide with iris coupled end plates, operating in TE_{10n} mode. For a dielectric measurement the sample should be placed in a maximum electric field and for a magnetic measurement, in a maximum magnetic field.

The figure above shows a rectangular cavity resonator inserted with a sample. If the sample is inserted through a hole in the middle of the waveguide length, then an odd number of half wavelengths (n=2k+1) will bring the maximum electric field to the sample location, so that the dielectric properties of sample can be measured.