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

Prof. M J Akhtar and Prof. K V Srivastava   

In this experiment, one can measure the dielectric properties of the sample material using Cavity Perturbation Tecnique. This technique is applicable for low loss samples and the volume of the sample inserted should be very small in comparison to the volume of the empty cavity as shown in the figure below.


S21 vs frequency plots

When a sample is inserted in the cavity there is shift in the resonant frequency (fr) and the quality factor (Q) of the cavity. With the change in these parameters, dielctric conatant ε'r and loss facor ε"r of the material can be calculated.


    Qo is the quality factor of empty cavity
    Qs is the quality factor of sample material inserted in the cavity
    fc is the resonant frequency of empty cavity
    fs is the resonant frequency of the sample inserted in the cavity resonator