A Multistep Approach for Accurate Permittivity Measurements of Liquids Using a Transmission Line Method

High-precision permittivity measurements of liquids are essential for specifying the specific absorption rate (SAR) of radio-frequency power absorbed by the human tissue. At Van Swinden Laboratorium (VSL), a commercial unit is used for these measurements, which uses a combination of transmission and reflection methods. The permittivity of unknown liquids is derived from the measured different scattering parameters of a transverse electromagnetic cell when the cell is empty and when it is filled with liquid. The flat slab structure of the cell makes it convenient for liquid filling and cleaning. The method is an efficient complementary approach for permittivity measurements compared with more complex setups in other national metrology institutes, but for reaching a low uncertainty, some inherent disadvantages need to be overcome. In this paper, a multistep approach is proposed using a combination of curve fitting, nonlinear progressive technique, and improved transmission line models in order to improve the uncertainty of the permittivity measurements. This approach is applicable even without a priori knowledge of the liquid properties. Using this approach, the useful frequency range of the commercial unit is successfully extended from 200 MHz-3 GHz to at least 100 MHz-4 GHz, and the accuracy is significantly improved to the level where it becomes comparable to more primary methods. The results obtained with this new method agree very well with the reference data from the literature.