Determination of Probe Position for Measurement of Liquid Permittivity with Open-ended Coaxial Probe

The permittivity of liquid is often measured with open-ended coaxial probe based on the single-ended reflection method. The volume of the liquid sample, in particular the probe immersion depth and the thickness of the liquid (i.e. the distance from the bottom of the liquid to the end of the probe) are the key factors affecting measurement results. In this paper, through FEM (Finite Element Method) simulation analysis and experimental measurements on typical liquid materials, for the first time, we have analyzed and established models for the insertion depth of the open-ended coaxial probe and the liquid thickness. And based on it, the effective insertion depth of the probe and the liquid thickness were determined. The analytical results show that the depth of the probe below the surface of the liquid is not less than 4 mm and the thickness of the liquid is not less than 7 mm for the measurement of the dielectric constant of the liquid material using an open-ended coaxial probe in the range of 0.5 GHz to 18 GHz, which provides a guideline to accurately measure the permittivity of liquids.