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. (c) 2025 Society of Photo-Optical Instrumentation Engineers (SPIE)