Near-Field FMCW Radar for Mapping of Multilayer Structures

Abstract

Near-field microwave non-destructive evaluation and inspection (NDE&I) of structures and materials is widely used in many applications. Single frequency measurements were found to be effective in detecting defects and abrupt changes, such as cracks and air voids, within homogenous structures. However, in some complex structures with many variables, multiple-frequency measurements were implemented to solve problems associated with more unknown parameters. These problems involved layered structures with more diversity in terms of number of factors that define the structure, namely, the layers thicknesses and relative permittivities ([epsilon]r). 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. Generally, from microwave's point of view, the interaction of the radiation from open-ended rectangular waveguides (RWGs) with any object can be modeled as radiations passing through multiple flat layers -The validity of the model is conditioned on the severity of planar variations of the modeled layers relative to the wavelength. If such a model is developed, it might enable us to measure the dielectric properties and thicknesses of the existing layers. A long term speculation of such application could be a microwave device that is capable of taking cross sectional images of the human body as a cheaper and more accurate replacement for the Magnetic Resonance Imaging (MRI) machine. In this thesis a novel technique on modeling the forward problem of near-field FMCW is presented. The technique is based on modeling the near-field frequency response of a multilayered structure illuminated by an open-ended RWG. The main objective of this research is to predict the return of an applied FMCW signal from a multilayered structure within the near-field of an open-ended RWG. The modeled forward problem opens new doors to develop a new microwave NDE&I approach for inspecting the integrity of layered structures with potentially high resolution.