FEM Analysis of Conical Type Coaxial Open-ended Probe for Dielectric Measurement

摘要

This paper deals with a numerical modelling technique based on finite elements method (FEM) in microwave frequency for computing the dielectric constant and loss factor of a homogeneous dielectric material by using conical-type coaxial probe. This sort of probe can be easily inserted into a wide range of biological tissue types and semi-rigid materials like rubber, some plastics, and organic materials (ex: dairy, butter, etc. for measuring moisture content). Where his feature is considered to be very important in biological and industrial applications. The measuring principle is based on detecting conductance and capacitance change with respect to the dielectric mass movement in the fringe electrical field. A three-dimensional finite-element formulation is employed in the dielectric material region and a small neighbouring region of the probe structure on which it is mounted. The electrical input admittance as well as the reflection coefficient are found from the finite-element analysis. From this, the conductance and capacitance related to the conical probe and fringing field are calculated and consequently the dielectric constant and loss factor are determined. In contrast to many other modelling techniques used for coaxial probe which are approximate and hence limited, the finite-element model is more exact and is applicable to complicated geometries. To demonstrate the accuracy of the numerical model, a parallel experimental study was carried out in the laboratory for the same geometric dimensions. The two results are compared and show excellent agreement, and also demonstrating that finite-element modelling is a good approach for optimized conical coaxial probe design.