Free-space millimeter-wave measurement of the complex dielectric constant of radome materials using nonlinear least-squares analysis

Describes a numerical optimization procedure based on the Powell (1965) nonlinear regression algorithm for measuring the complex dielectric constant of radome composite materials and coatings at millimeter wavelengths using free-space measurements of the transmission coefficient over a range of incidence angles and frequencies. Radomes usually need rain-erosion, antistatic, and hydrophobic coatings for environmental protection. In the present approach, the determination of the permittivity of these thin radome coatings is achieved by depositing the coating on a substrate of known dielectric constant and analyzing the transmission coefficient of the two-layer medium. Since the regression algorithm is used to analyze a large number of data points, it reduces the random errors in the experiments and yields more accurate information than the single-point free-space measurement methods used by other authors at millimeter wavelengths. The validity and accuracy of this technique were confirmed by making measurements on standard dielectrics such as polyethylene, nylon, and Teflon.<>