A number of diffraction problems which have practical applications are examined using the Wiener-Hopf-Hilbert technique. Each problem is formulated as a matrix Wiener-Hopf equation, the solution of which requires the factor~sation of a matrix kernel. Since the determinant of the matrix kernel has poles in the cut plane, the Wiener-Hopf-Hilbert technique is modified to allow the usual arguments to follow through. In each case an explicit matrix factorisation is carried out and asymptotic expressions for the field scattered to infinity are obtained. The first problem solved is that of diffraction by a semi-infinite plane with different face impedances. The solution includes the case of an incident surface wave as well as an incident plane wave for an arbitrary angle of incidence. Graphs of the far-field are provided for various values of the half-plane impedance parameters. The second problem examined is diffraction by a half-plane in a moving fluid. This is solved without restriction on the impedance parameters of the half-plane and includes both the leading edge and trailing edge situations. The final problem is of radiation from an inductive wave-guide. Expressions are obtained for the field radiated at the waveguide mouth and the field reflected in the duct region.

No description available.

No description available.

Dissertation (Ph.D.) --University of Texas Graduate School of Biomedical Sciences at San Antonio, 2008. / Vita. Includes bibliographical references.

<p> On August 21, 2017 a total solar eclipse occurred over the United States commencing on the west coast moving across to the east coast providing an opportunity to observe how the rapid day-night-day transition changed the ionosphere&rsquo;s D-region electron density and how very low frequency (VLF) electromagnetic wave propagation was affected. To observe the solar obscurity effects, VLF receivers were deployed in two locations: one in the path of totality in Lakeside, Nebraska and another south of the totality path in Hugo, Colorado. The locations were chosen to achieve an orthogonal geometry between the eclipse path and propagation path of U. S. Navy VLF transmitter in North Dakota, which operates at 25.2 kHz and has call sign NML. VLF amplitude and phase changes were observed in both Lakeside and Hugo during the eclipse. A negative phase change was observed at both receivers as solar obscuration progressively increased. The observed phase changes became positive as solar obscuration reduced. The opposite trend was observed for the amplitude of the transmitted signal: growth as max totality approached and decay during the shadow&rsquo;s recession. The Long Wave Propagation Capability (LWPC) code developed by the US Navy was used to model the observations. LWPC is a modal solution finder for Earth-ionosphere waveguide propagation that takes into account the D-region density profile. In contrast to past efforts where a single ionosphere profile was assumed over the entire propagation path, a degree of spatial resolution along the path was sought here by solving for multiple segments of length 100-200 km along the path. LWPC modeling suggests that the effective reflection height changed from 71 km in the absence of the eclipse, to 78 km at the center of the path of totality during the total solar eclipse and is on agreement with past work.</p><p>

In the summer of 2007, magnetic and electromagnetic conductivity surveys were performed on the third (front) terrace of Monks Mound, Cahokia, 6 miles (~10 km) east of St Louis in Illinois. A 17 m by 40 m rectangular grid was established on the third terrace of the 850 year old mound surface. Readings were taken on the rectangular grid at one meter spacing. A Geometrics G-856 proton precession magnetometer and a Geonics EM38 conductivity meter were used for the survey. The survey was designed and executed to detect anomalous features within the very shallow subsurface (one to two meters deep). It was hoped to delineate and categorize the sources of the anomalies. The causes are known to be natural (in this case limited to lightning strikes), prehistoric (pottery, kilns, hearths), historic (metal, fire pits, structure remnants), or modern (metal). The larger, better-defined anomalies were numbered and discussed. Magnetic anomaly interpretations include a root cellar (4), outhouse (10), house (6), and metal features (7, 8, 14, 15). The certainty of the interpretations varied and there was often multiple interpretations possible. The EM anomaly interpretations proposed include metal, pit, grave, and animal burrowing. Good correlation is observed between some of the anomalies for the magnetic and electromagnetic methods, while other anomalies were observed using only one method. Further testing suggested to confirm the interpretations is outlined in the thesis.

abstract: The micromotions (e.g. vibration, rotation, etc.,) of a target induce time-varying frequency modulations on the reflected signal, called the micro-Doppler modulations. Micro-Doppler modulations are target specific and may contain information needed to detect and characterize the target. Thus, unlike conventional Doppler radars, Fourier transform cannot be used for the analysis of these time dependent frequency modulations. While Doppler radars can detect the presence of a target and deduce if it is approaching or receding from the radar location, they cannot identify the target. Meaning, for a Doppler radar, a small commercial aircraft and a fighter plane when gliding at the same velocity exhibit similar radar signature. However, using a micro-Doppler radar, the time dependent frequency variations caused by the vibrational and rotational micromotions of the two aircrafts can be captured and analyzed to discern between them. Similarly, micro-Doppler signature can be used to distinguish a multicopter from a bird, a quadcopter from a hexacopter or a octacopter, a bus from a car or a truck and even one person from another. In all these scenarios, joint time-frequency transforms must be employed for the analysis of micro-Doppler variations, in order to extract the targets’ features. Due to ample bandwidth, THz radiation provides richer radar signals than the microwave systems. Thus, a Terahertz (THz) micro-Doppler radar is developed in this work for the detection and characterization of the micro-Doppler signatures of quadcopters. The radar is implemented as a continuous-wave (CW) radar in monostatic configuration and operates at a low-THz frequency of 270 GHz. A linear time-frequency transform, the short-time Fourier transform (STFT) is used for the analysis the micro-Doppler signature. The designed radar has been built and measurements are carried out using a quadcopter to detect the micro-Doppler modulations caused by the rotation of its propellers. The spectrograms are obtained for a quadcopter hovering in front of the radar and analysis methods are developed for characterizing the frequency variations caused by the rotational and vibrational micromotions of the quadcopter. The proposed method can be effective for distinguishing the quadcopters from other flying targets like birds which lack the rotational micromotions. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2018

abstract: Multiport antennas offer greater design flexibility than traditional one-port designs. An antenna array is a special case of a multiport antenna. If the antenna's inter-element spacing is electrically small, the antenna is capable of achieving superdirectivity. Superdirective antenna arrays are known to be narrow band and have low radiation resistance which leads to low radiation efficiency and high VSWR. However, by increasing the self-impedance of the antenna elements, the radiation resistance is increased but the bandwidth remains narrow. A design methodology is developed using the ability to superimpose electric fields and multi-objective optimization to design antenna feed networks. While the emphasis in this dissertation is on antenna arrays and superdirectivity, the design methodology is general and can be applied to other multiport antennas. The design methodology is used to design a multiport impedance-matching network and optimize both the input impedance and radiation pattern of a two-port superdirective antenna array. It is shown that the multiport impedance-matching network is capable of improving the input impedance of the antenna array while maintaining high directionality. The antenna design is critical for the methodology to improve the bandwidth and radiation characteristics of the array. To double the bandwidth of the two-port impedance matched superdirective antenna array, a three-port Yagi-Uda antenna design is demonstrated. The addition of the extra antenna element does not increase the footprint of the antenna array. The design methodology is then used to design a symmetrical antenna array capable of steering its main beam in two directions. / Dissertation/Thesis / Ph.D. Electrical Engineering 2012

abstract: The fluorescence enhancement by a single Noble metal sphere is separated into excitation/absorption enhancement and the emission quantum yield enhancement. Incorporating the classical model of molecular spontaneous emission into the excitation/absorption transition, the excitation enhancement is calculated rigorously by electrodynamics in the frequency domain. The final formula for the excitation enhancement contains two parts: the primary field enhancement calculated from the Mie theory, and a derating factor due to the backscattering field from the molecule. When compared against a simplified model that only involves the primary Mie theory field calculation, this more rigorous model indicates that the excitation enhancement near the surface of the sphere is quenched severely due to the back-scattering field from the molecule. The degree of quenching depends in part on the bandwidth of the illumination because the presence of the sphere induces a red-shift in the absorption frequency of the molecule and at the same time broadens its spectrum. Monochromatic narrow band illumination at the molecule's original (unperturbed) resonant frequency yields large quenching. For the more realistic broadband illumination scenario, we calculate the final enhancement by integrating over the excitation/absorption spectrum. The numerical results indicate that the resonant illumination scenario overestimates the quenching and therefore would underestimate the total excitation enhancement if the illumination has a broader bandwidth than the molecule. Combining the excitation model with the exact Electrodynamical theory for emission, the complete realistic model demonstrates that there is a potential for significant fluorescence enhancement only for the case of a low quantum yield molecule close to the surface of the sphere. General expressions of the fluorescence enhancement for arbitrarily-shaped metal antennas are derived. The finite difference time domain method is utilized for analyzing these complicated antenna structures. We calculate the total excitation enhancement for the two-sphere dimer. Although the enhancement is greater in this case than for the single sphere, because of the derating effects the total enhancement can never reach the local field enhancement. In general, placing molecules very close to a plasmonic antenna surface yields poor enhancement because the local field is strongly affected by the molecular self-interaction with the metal antenna. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013

Efficient computation in deterministic and uncertain electromagnetic propagation environments, tackled by parabolic equation methods, is the subject of interest of this dissertation. Our work is comprised of two parts. In the first part we determine efficient absorbing boundary conditions for propagation over deterministic terrain and in the second part we study techniques for efficient quantification of random parameters/outputs in volume and surface based electromagnetic problems. Domain truncation by transparent boundary conditions for open problems where parabolic equation is utilized to govern wave propagation are in general computationally costly. For the deterministic problem, we utilize two approximations to a convolution-in-space type discrete boundary condition to reduce the cost, while maintaining accuracy in far range solutions. Perfectly matched layer adapted to the Crank-Nicolson finite difference scheme is also verified for a 2-D model problem, where implemented results and stability analyses for different approaches are compared. For the random problem, efficient moment calculation of electromagnetic propagation/scattering in various propagation environments is demonstrated, where the dimensionality of the random space varies from N = 2 to N = 100. Sparse grid collocation methods are used to obtain expected values and distributions, as a non-intrusive sampling method. Due to the low convergence rate in the sparse grid methods for moderate dimensionality and above, two different adaptive strategies are utilized in the sparse grid construction. These strategies are implemented in three different problems. Two problems are concerned with uncertainty in propagation domain intrinsic parameters, whereas the other problem has uncertainty in the boundary shape of the terrain, which is realized as the perfectly conducting (PEC) Earth surface.