Microwave Nondestructive Testing (MNDT) techniques have advantages over other NDT methods (such as radiography, ultrasonics, and eddy current) regarding low cost, good penetration in nonmetallic materials, good resolution and contactless feature of the microwave sensor (antenna). For MNDT techniques, the measured parameters are reflection coefficients, transmission coefficients, dielectric constants, loss factors, and complex permeabilities as a function of microwave frequency and temperature. These measured parameters can be related to material parameters of interest (e.g., flaws, binder content, moisture content, etc.) by suitable modeling and calibration. We have employed a free-space microwave measurement (FSMM) system which can measure electromagnetic properties (complex permittivity, complex permeability, reflection coefficients, etc.) for evaluation of composite materials. The main advantage of this FSMM system is that with suitable modifications, it is possible to make precise, accurate and reproducible MNDT measurements on composite materials under high or low temperature conditions and complex electromagnetic environmental conditions (e.g., DC biasing fields, ionizing radiation, etc.) due to contactless feature of free-space measurements. This measurement system consists of a pair of spot-focusing horn lens antennas, mode transitions, coaxial cables and a vector network analyzer (VNA). The inaccuracies in free-space measurements are due to two main sources of errors. 1) Diffraction effects at the edges of the material specimen. 2) Multiple reflection between horn lens antennas and mode transitions via the surface of the sample. The spot-focusing antennas are used for minimizing diffraction effects and free-space LRL (line, reflect, line) calibration method implemented on VNA eliminates errors due to multiple reflections. In this paper, we have used free-space implementation of reflection-transmission method for simultaneous determination of complex permittivity (ε~*) and complex permeability (μ~*) of magnetic materials. ε~* and μ~* values are reported for carbonyl iron loaded silicon rubber sheets with carbonyl iron concentration varying from 20% to 50% (by volume).
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