Phase-shift ambiguity in microwave dielectric properties measurements

Phase measurements of the transmission coefficient are important when used for the dielectric characterization of materials. They are required for industrial material monitoring applications, where the phase is correlated with parameters such as moisture content and density, which need to be continuously determined. However, when the thickness of the material under test is greater than the wavelength in the material, a phase problem is encountered. Two methods are proposed to solve this problem. The first is based on the selection of the appropriate material thickness; the second requires the use of measurements at two frequencies. Advantages and limitations of both methods are discussed, and numerical validations are given for particulate materials.