Nondestructive EM Characterization of Two-Layer Absorbers Using Coupled Coaxial Probes

A measurement system for nondestructive electromagnetic characterization of two-layer absorbers made of a dielectric layer on top of a lossy magnetic layer is presented. The characterization includes simultaneous determination of the complex permittivity of the first layer and the complex permittivity and permeability of the second layer over a broad frequency bandwidth. The measurement system is composed of two flanged open-ended coaxial probes coupled to each other via a two-layer absorber. A full-wave analytical model is worked out to determine the theoretical reflection and transmission coefficients. The material properties of each layer are extracted by solving the inverse problem via nonlinear least squares. Numerical and experimental results for a two-layer absorber are reported and compared with those of standard 7 mm coaxial measurements to validate the proposed approach.