Near-Field Analysis of Rectangular Waveguide Probes Used for Imaging

Abstract

Near-field microwave imaging of composite structures has received considerable attention recently. The success achieved on the experimental level motivated the development of a theoretical model to describe the high quality images obtained using near-field microwave imaging [1–4]. This theoretical model will also help in building an intuitive understanding of the behavior of the fields inside dielectric materials in the near-field of an open-ended rectangular waveguide probe. A near-field microwave image is the result of several factors such as probe type (example rectangular waveguide, circular waveguide or coaxial line), field properties (i.e. main lobe, sidelobes and half power beam width, etc.), geometrical and physical properties of both the defect and the material under inspection. Thus, in order to characterize a defect, the effect of all non-defect factors needs to be taken out of an image. One of the dominant non-defect factors which influences an image significantly is the radiator field properties. Thus, it is essential to formulate the properties of the fields radiating out of an open-ended rectangular waveguide in its near-field. This knowledge will aid in formulating the forward problem when imaging a defect, and will be used to solve the inverse problem for obtaining defect properties. In this paper fields radiating out of an open-ended rectangular waveguide, into an infinite half-space of a dielectric material, are calculated and used to explain some of the features observed in experimental near-field microwave images.