A new in situ procedure for measuring the dielectric properties of low permittivity materials

In situ measurements of the dielectric properties of relatively km, complex permittivity materials such as rocks and fatty tissues have recently become of significant interest in industrial and medical applications. The presently available in vivo probes although found adequate for measuring the dielectric properties of high-permittivity material, such as a high-permitivity materials, such as high dielectrical ttissues [1], [2], were not suitable for accurately measuring the properties of low-permittivity materials. For low-permittivity substances, the measurement procedure and the accuracy of the obtained results were found to significantly improve when the length of the center conductor (A) of the coaxial transmission line probe [1], [2] was further extended into the material under test. With such an extension, the approximate analysis used in the previous measurements (1], (2) were invalid, and hence a new measurement procedure was developed. This new procedure has the following features: 1) it utilizes a rigorous expression for the input impedance of the in situ probe which accurately accounts for the radiation resistance of the probe for larger values of (h/λ); 2) the dielectric parameters of the sample under test were determined by comparing the measured and calculated values of the input impedance using an iterative two-dimensional (error surface) complex zero-finding routine. In this paper, the new measurement procedure is described, results of the error (uncertainty) analysis are presented, and the accuracy of the measurement procedure is illustrated by presenting dielectric results for known lossy (octyl alcohol) and lossless (heptane) low-permittivity materials.