Measuring the refractive index of materials with very low reflection coefficients at millimeter wavelengths

Structural materials that are virtually invisible during millimeter-wave imaging are needed for applications in testing advanced imaging technology screening systems, for example, by supporting image-quality test objects. Laboratory measurement of the electrical permittivity of candidate materials at the frequency of the imaging system can appraise their suitability as very low-reflective materials, but the measurement is challenging because the ideal material has a real component of permittivity near unity and nearly zero propagation loss. A method is described based on temporal features in wave packet propagation using dual calibrations for front and back interfaces. Measurements of different foam materials are demonstrated.