Multilayer medium technique for nondestructive EM-properties measurement of radar absorbing materials using flanged rectangular waveguide sensor and FDTD method

The mathematical truth associated with the simultaneous multiparameter measurement using reflection only sensors is that if there are n unknowns to be determined, we need at least n independent measurements to be able to solve even one of them. In this paper, multilayer medium technique is employed to produce the needed independent reflection coefficients for simultaneous nondestructive determination of complex permittivity and permeability of radar absorbing coatings at X-band of microwave frequency range. The technique is based on measuring two reflection coefficients of finite flange open-ended waveguide reflection sensor placed in close contact with the material under test backed by metal. The first reflection coefficient is measured using sample of material under test, while the second one is done to test again a combination of this sample followed by material sample with a known permittivity, permeability and thickness to form a multilayer medium sample. Finite-Difference Time-Domain (FDTD) method is applied to numerically formulate the aperture admittance and calculate reflection coefficients for the two cases. Both complex permittivity and permeability are extracted iteratively by imposing reflection coefficient value, both obtaining from FDTD modeling and measurement using Newton-Raphson method. The technique is used to measure EM-properties of radar absorbing coatings using Vector Network Analyzer. Preliminary results of ε* and μ* are in good agreement with published data. Through numerical simulations and measurement, it has demonstrated that the technique is promising for simultaneous nondestructive multiparameter (EM-properties and physical quantities) measurement of high loss materials and other applications such as thickness evaluation of layered media. The FDTD simulations and experiments results are presented.