Multimodal Solution for a Rectangular Waveguide Radiating into a Multilayered Dielectric Structure and its Application for Dielectric Property and Thickness Evaluation

Open-ended rectangular waveguides are widely used for microwave and millimeter wave nondestructive testing applications. Applications have included detecting disbonds and delaminations in multilayered composite structures, thickness evaluation of dielectric sheets and coatings on metal substrates, etc. When inspecting a complex multilayered composite structure, made of generally lossy dielectric layers with arbitrary thicknesses and backing, the dielectric properties of a particular layer within the structure is of particular interest, such being health monitoring of structures such as radomes. The same is also true where one may be interested in the thickness or more importantly thickness variation of a particular layer within such structures. An essential tool for estimating the dielectric constant or thickness is an accurate model for simulating reflection coefficient at the aperture of the probing open-ended waveguide. One issue of interest is that radiation from open-ended rectangular waveguides into layered dielectric structures has been considered only when accounting for the dominant waveguide mode. However, when using these models for recalculating dielectric constant or thickness, the results may not be accurate (depending on the measurement requirements). To this end, this paper provides an accurate model for the reflection coefficient which also accounts for the effect of higher-order modes. Finally, the potential of this model for accurately estimating dielectric constant is shown.