Free-Space Permittivity Measurements on Dielectric Materials at Millimeter Wavelengths

A system is described for measuring the complex permittivities of rigid dielectric sheet materials at millimeter wavelengths, using free-space transmission techniques and Brewster angle determinations. A first-order analysis is given for the effect of multiple reflections in thick low-loss dielectric sheets, under conditions of oblique incidence. Values of ϵr and tan δ are given for selected sheet materials at 35 GHz and 25°C, that are estimated to be accurate to ±5 percent.