A New Method for Measuring Permittivity Using Terminated Rectangular Waveguide

A new method for measuring the permittivity of liquid and powder sample using a rectangular X-band waveguide is described. One end of the waveguide is terminated and a transition terminal is connected to the other end. A sample is packed in the waveguide. An X band wave is transmitted to the waveguide and a reflected wave is detected through a directional coupler connecting to the transition. The reflected wave is fed to a network analyzer by which the amplitude and the phase of the reflected wave are measured. The Permittivity of the sample is obtained from the ratio of the reflectances with and without the sample. The range of thickness of the sample is chosen so as to have the optimum sensitivity. Thus, the reflectance is measured by changing the thickness of sample. The permittivity is calculated from the rms method using the change of reflectance with thickness. There are several advantages in this method. Because TE10 mode within the waveguide is maintained correctly, accurate relationship between the reflectance and the permittivity is established, and the error can be reduced by adjusting the thickness of samples. The propriety of this method was confirmed by measuring the liquid samples the permittivity of which is known. Powder samples were measured and good reproducibility was obtained.