Multimodal Solution for a Waveguide Radiating Into Multilayered Structures—Dielectric Property and Thickness Evaluation

Open-ended rectangular waveguides are widely used for microwave and millimeter-wave nondestructive testing (NDT) applications, such as detecting disbond and delamination in multilayered composite structures, thickness evaluation of dielectric sheets and coatings on metal substrates, etc. When inspecting a complex multilayered composite structure that is made of generally lossy dielectric layers with arbitrary thicknesses and backing, the dielectric properties of a particular layer may be of particular interest (e.g., radome inspection). The same is also true when one is interested in the thickness, or, more importantly, thickness variation, of a particular layer within such complex structures. An essential tool for closely estimating the complex permittivity and/or thickness is an accurate forward electromagnetic model for simulating the reflection coefficient at the aperture of the probing open-ended waveguide. To this end, this paper provides a full-wave accurate forward model for calculating the reflection coefficient from a generally lossy multilayered composite structure possessing an arbitrary number of layers and respective thicknesses while accounting for the influence of higher order modes. This model is subsequently validated through comparisons with a commercial numerical tool and actual measurements. Furthermore, a measurement model is provided, which results in an iterative inverse technique for estimating the complex permittivity and thickness of a dielectric layer. Subsequently, this technique is applied to the measured reflection coefficients of several structures. To evaluate the accuracy of this technique, an analysis on its sensitivity to various sources of errors, and, most importantly, the effect of a finite flange size, is also demonstrated by using the simulated data. Finally, the potential of this model to accurately estimate the thickness of an individual layer, which represents a thin disbond, in a multilayered composite structure is presented.