Synthesized pulse reflection by non-smooth surfaces in 8-mm wave range

Abstract

Composite dielectric structures are widely used as materials for solid-motor cases, thermal protection structures and sandwich radomes. Estimation of thickness and dielectric constant of every layer is very important for mechanical and radiophysical property prediction of the product. Microwaves provide non-contact measurements. The direct way to obtain reflection from the surface separately from other spurious reflections is to use time-domain reflectometry. For waveguide measurements it is more preferable to measure the reflection coefficient at a set of frequencies and transform experimental data from frequency to time domain using one of the spectral numerical algorithms. This approach is similar to time pulse probing. For plane surfaces, it was shown that the ratio of amplitudes of peaks for front interface of structure and metal plate was equal to Fresnel coefficient. The real machinery products of dielectrics have rough surfaces and cylindrical and spherical form. Many products of dielectrics have regular surface structure. Correct values of Fresnel reflection coefficient and the value of dielectric constant reconstructed are necessary for correct estimation of geometrical thickness of every layer. This situation induces some experimental problems in millimeter wave range scattering. The complexity of the situation is stipulated by properties of near-field radiation from real constructions of an open-ended waveguide.