Calibration Procedure for Complex Permittivity Extraction Using an Open-Ended Coaxial Probe

More Info


As a part of the development of a device for skin cancer tissue detection, this report is about the development of a new calibration procedure for measuring complex permittivity with an open-ended coaxial probe. The developed method is based on comparing the measured reflection coefficient to an array of reference coefficients for different complex permittivities. The implementation consists of the following parts: - Correct Vector Network Analyser setup and readout from 0.5 to 20 GHz; - Simulations to generate the reference data for relative real permittivities from 1 to 41 and relative imaginary permittivities from 0 to 15; - Calibration using a 3-term error model; - The permittivity lookup algorithm, which matches a given load reflection coefficient ΓL to the corresponding complex permittivity. The program works well but the quality of the results depends on the accuracy of the 3-D EM simulation model which generates the lookup table data. This is visible in the relative errors that are decreasing with higher frequencies. The set of materials under test consisted of three liquids: butanol, methanol and propanol, and two skin depicting phantoms: with 30 % and 50 % oil. The estimation errors of the liquid permittivities are often below 1, but have peaks up to 5 at lower frequencies. The skin depicting phantom with 30 % oil turned out to have a non-uniform structure and gave the most variation in permittivity results. The permittivity results could be improved to be smoother by implementing better interpolation methods for complex numbers. The execution time of the program has been found to be 1058 ms, which is deemed fast enough for the targeted application, but could still be improved by making a better trade-off between speed and accuracy.