Noninvasive measurement of dielectric properties in layered structure: A system identification approach

In this paper, estimation of the dielectric properties of layered lossy structure using open-ended coaxial probes is analyzed. Theoretical and empirical exponential approximations of the relaxation process are generalized for two, three and four-layers structures. As measurements are taken in the 10 MHz to 1 GHz frequency range, a wide frequency spectrum measurement procedure is proposed, providing a fast method to approximate parameters (thickness and dielectric properties) of each layer. Application of the methodology depends on some prior knowledge of the properties of the dielectric layers (i.e. number of layers and approximate values of their thicknesses and permittivities). A linear System Identification method is proposed using time domain measurements to find the corresponding frequency responses. Although these approaches and techniques have been already considered individually, they are combined here, resulting in a novel methodology to process time domain reflectometry data that is robust and numerically well-behaved. Simulations and experimental results in phantom and a biological tissue are provided. Dielectric relaxation is assumed to follow a Debye model, but comments on other parameterizations are also summarized. Experimental frequency validation data are reliable up to 700 MHz.