Near-field microwave model of multilayered structures illuminated by open-ended rectangular waveguides

Near-field microwave non-destructive evaluation and inspection of structures and materials is widely used in various applications. Single-frequency measurements were found to be effective in detecting defects and abrupt changes, such as cracks and air voids, within homogeneous structures. However, in some complex structures with many variables, multiple-frequency measurements were utilised to solve problems associated with more unknown parameters. Those problems involved layered structures with more diversity in terms of number of factors that define the structure, namely, the thicknesses and relative complex permittivities (ε_r) of the layers. Later on, more systematic broadband techniques, such as frequency-modulated continuous wave, proved to be effective in measuring thicknesses of layers in a multilayered structure. However, obtaining permittivity information was still a big challenge. This study presents a novel analytical near-field model that relates the radiation of a rectangular waveguide with the reflection when applied to a dielectric multilayered structure. The input-output relation is characterised by the frequency response of the structure which was analytically derived. As a side result the distribution of the reflection coefficient over the aperture was also analysed and the expression was derived. The main goal is to define the relationship between the structure layers, in terms of thicknesses and relative permittivities and the aperture-s complex reflection coefficient (Γ) for all frequencies of operation. This model can be used in developing new microwave non-destructive evaluation techniques for inspecting the integrity of layered structures with potentially high resolution.