Analysis of Open-Ended Coaxial Probes by Using a Two-Dimensional Finite-Difference Frequency-Domain Method

A 2-D finite-difference frequency-domain (FDFD) method is presented to analyze open-ended coaxial probe problems. With the axial symmetry of geometry taken into account, the method reduces the original structure into an equivalent 2-D problem. Due to its ability to handle complex geometries, the presented method can tackle many practical situations when the analytical/semianalytical full-wave analysis is unfeasible. Numerical examples involving the infinite-flanged probes and the finite-flanged probes are included. In the examples of the infinite-flanged probes, the accuracy of the 2-D FDFD method and of the 2-D finite-difference time-domain (FDTD) method are checked with the results of the semianalytical full-wave analysis method. It is found that the accuracy of the 2-D FDFD method and of the 2-D FDTD method are the same, as long as their mesh structures are the same. In the examples of the finite-flanged probes, whose analytical/semianalytical full-wave analysis solutions are unfeasible, the results of the 2-D FDFD and 2-D FDTD methods are found to be in very good agreement. All the numerical examples show that the computational efficiency of the 2-D FDFD method is much higher than that of the 2-D FDTD method.

I. Introduction

Open-ended coaxial probes are widely used for noninvasive or nondestructive determination of electromagnetic (EM) properties of materials in a wide frequency range, where the TEM mode is the only propagating mode inside the probes [1]–[4]. The basic principle of the measurement is to measure the reflection coefficient of the fundamental mode, i.e., the TEM mode, when the probe is terminated by the material under test (MUT). The EM properties of the MUT can then be determined by an appropriate inverse process. Since the reflection coefficient of the fundamental mode contains valuable information about the terminating material, the probes are also useful sensing tools that are used to accurately measure the thickness of thin dielectric slabs, to detect the presence of disbonds and delaminations in layered media [5], and to detect skin cancer [6].