Open-ended rectangular waveguide near-field frequency response of multilayered structures

Near-field microwave nondestructive evaluation and inspection (NDE&I) of structures and materials is widely used in various applications. Single frequency measurements are used effectively in detecting defects and abrupt changes, such as cracks and air voids, within structures, composites, surfaces, etc. These measurements were quite effective when applied on homogenous structures. However, in more complex structures with many variables, it was imperative to use multiple-frequency measurements to solve the 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 (FMCW) proved to be effective in measuring thicknesses of layers in a multilayered structure. However, obtaining permittivity information was still a big challenge. This paper presents a novel technique on modelling the near-field frequency response of a multilayered structure illuminated by an open-ended rectangular waveguide (RWG). The main goal is to derive 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 NDE&I techniques for inspecting the integrity of layered structures with potentially high resolution.