Permittivity Determination using Time-Filtered Data from Antenna-Near-Field Measurements on Bounded Structures

Results of free space measurements on plane dielectric layer-structures are influenced by boundary inhomogeneities, if the structure is bounded. This influence may be eliminated by time-filtering, which often requires a configuration, where the dielectric surface is no more situated in the antenna far-field. Under this condition a normalization of the results with respect to those for a metallic plate is proposed. With regard to these normalized reflection-data, two conclusions are presented: The term owing to the first reflection at the first interface is closely approximated by the simple relation for a plane TEM-wave with normal incidence. The term which belongs to the first reflection at the second interface may differ significantly from the corresponding TEM-value, but can be corrected approximately by a known factor. Once those terms, taking the influence of the radiation-field into account, are known, the standard TEM-methods for permittivity-evaluation may be applied.