Multiple reflection of a microwave beam between a horn antenna and a smooth target in the antenna near field produces an interference effect in the received power. This effect can be described by modeling the antenna and target as a closely coupled system with ‘‘coherent’’ reflection coefficients. This model shows better agreement with experimental data than models based on isotropic antenna ‘‘backscatter.’’ Measurement techniques are derived from this new model, which permit, by sampling the interference pattern, the remote sensing of the reflection coefficient of an unknown surface. From the reflection coefficient the dielectric constant or surface roughness can be obtained. A simple diagnostic apparatus is described together with possible industrial/laboratory applications.