Waveguide permittivity measurement using variable-length samples and an uncalibrated reflectometer

A method is presented to measure the complex permittivity of samples inserted into waveguides. The method is based on reflection coefficient measurements for four different sample lengths, which are used to determine the complex propagation constant of the guide's fundamental mode. For simple waveguide geometries, the complex permittivity can be calculated, assuming a value of 1 for the relative permeability. Unlike classical waveguide techniques, this method does not require calibration of the reflectometer, and it can be used with samples that are narrower than the waveguide cross section's main dimension. By using narrower samples, the upper frequency limit of the method can be increased. Measurements in an X-band rectangular waveguide using the proposed method are presented and compared with results obtained with a cavity perturbation method. Very good agreement has been found over the frequency range prescribed for the method. Results are shown for completely filled and partially filled waveguides.