Electromagnetic waves--Scattering

In shallow water or fluid half-space, the acoustic scattering from a target is significantly different from that of an unbounded medium, due to the multiple reflections occurring between the target and the boundaries. The purpose of this thesis is to investigate the influence of the boundaries on the acoustic scattering of a rigid sphere by means of a superposition method. A minimum number of point sources necessary to accurately model the scattered field is determined in the case of a free medium, a fluid half-space and a waveguide. The free field symmetry vanishes due to the presence of boundaries and, at particular frequencies or scatterer depths, a significant change in the magnitude and spatial distribution of the scattered field occur. In an unbounded medium or fluid half space, the superposition method is shown to give similar results to analytical formulations found in the literature, provided enough point sources are used.

This research addresses a specific class of electromagnetic problems concerning the radiation and scattering of high frequency electromagnetic waves at the surfaces of composite materials. With the advent of need-based developments in electromagnetic material technology, a research niche has stemmed to analyze the interaction of electromagnetic energy with different versions of composite materials used mostly as surface materials such as in radar-stealth applications. Mixture-dielectrics, mixture magnetic materials, textured electromagnetic composites with matrix layers of lossy dielectric/magnetic materials, chiralic media, active surface materials etc. are a few emerging candidates of viable composites being considered in the state-of-the-art engineering electromagnetics. Specific to these materials, the analyses pertaining to electromagnetic radiation and scattering problems require a unique, approach vis-a-vis the heterogeneous properties of the composite material surfaces involved. Presently, the proximity of such surfaces is characterized and duly accounted for, by a mutual immittance formulation based on the Monteath's field compensation theorem. Using the relevant theoretical considerations, electromagnetic plane wave and/or focused beam radiation due to an aperture, conducting patch on flat and curved surfaces and scattering by an object coated with a composite material are elucidated. Also, an experimental method of evaluating the surface immittance is indicated. Theoretical computations are validated by comparing the results with those obtained via other methods. Some experimental results are furnished in support of the theoretical approaches presented.

The research addressed refers to a study on the electromagnetic performance aspects of body-worn radio units operating in the presence of scatterers in close proximity, using analytical, numerical, and experimental methods. The application potentials of such methods include evaluating the integrity of radio units such as cell phones. Consistent with the scope of the study above, considered in this research are specific details on analytical and numerical modeling of the effects of a nearby conducting cylindrical object on the electromagnetic field near a human-model phantom. Calculations are performed using the Finite Difference Time Domain (FDTD) method. Considered are various separations of the body wearing the test radio unit from the proximal object and polarization of the incident wave. An anechoic chamber and the test setup used for the measurement of EM field amplitudes near a saline-water phantom are described. Within the anechoic chamber, a small shielded loop is used as a field measurement probe and is positioned near the test phantom. The field probe orientation was in the vertical plane for characterizing the prevailing electromagnetic field intensity. This study indicates that variations in the field amplitude near the phantom occur, which are responsive to phantom rotation and measurement distance from the phantom. The electromagnetic field amplitude decreases rapidly with increasing distance between the probe and the surface of the phantom. The analysis is also extended to examine the electromagnetic field distribution in the gap between a human body phantom model and a nearby conducting cylinder. An appropriate three-dimensional FDTD method is presented and applied to a near-field problem of analyzing the influence of proximal conductive objects on fields near a phantom wearing an RF unit.