Effects of Calibration Process on Complex Dielectric Permittivity Measurements with Open-Ended Coaxial Probes

In this paper, we investigate the short-circuit calibration step of the open-ended coaxial probe (OECP) which is one of the major challenges when measuring complex dielectric permittivity (CDP) of a material under test (MUT). The effect of short-circuit inaccuracy can be mitigated by an alternative calibration approach by taking advantage of the calibration mathematical model of the OECP and using known liquid instead of the commonly used metallic short-circuit procedure. The CDP results of formamide and Dimethyl Sulfoxide (DMSO) as the MUT are compared using three known liquids acetone, butanol, and 2-propanol respectively, as the shorting material during the calibration procedure. The MUT and calibration liquids were simulated with Ansys HFSS simulation program from 0.5 to 6 GHz frequency range with a resolution of 0.25 GHz. The commercially available Speag DAK 3.5-mm OECP was replicated with similar material specifications and dimensions. In order to retrieve the CDPs from reflection coefficients (RCs) obtained from the probe-MUT interface, a multi-frequency Gauss-Newton inversion method based is implemented. The results demonstrate that it is important to select an appropriate calibration material during the calibration procedure to ensure reliable CDP results. The study also indicates that further numerical investigation is required to explain the relationship between the calibration mathematical model and CDP.