VTechWorks Repository :: Browsing by Author "Davis, William A."

Browsing by Author "Davis, William A."

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3D Micromachined Passive Components and Active Circuit Integration for Millimeter-wave Radar Applications
Oliver, John Marcus (Virginia Tech, 2012-02-20)
The development of millimeter-wave (30-300 GHz) sensors and communications systems has a long history of interest, spanning back almost six decades. In particular, mm-wave radars have applications as automotive radars, in remote atmospheric sensing applications, as landing radars for air and spacecraft, and for high precision imaging applications. Mm-wave radar systems have high angular accuracy and range resolution, and, while susceptible to atmospheric attenuation, are less susceptible to optically opaque conditions, such as smoke or dust. This dissertation document will present the initial steps towards a new approach to the creation of a mm-wave radar system at 94 GHz. Specifically, this dissertation presents the design, fabrication and testing of various components of a highly integrated mm-wave a 94 Ghz monopulse radar transmitter/receiver. Several architectural approaches are considered, including passive and active implementations of RF monopulse comparator networks. These architectures are enabled by a high-performance three-dimensional rectangular coaxial microwave transmission line technology known as PolyStrataTM as well as silicon-based IC technologies. A number of specific components are examined in detail, including: a 2x2 PolyStrata antenna array, a passive monopulse comparator network, a 94 GHz SiGe two-port active comparator MMIC, a 24 GHz RF-CMOS 4-port active monopulse comparator IC, and a series of V- and W-band corporate combining structures for use in transmitter power combining applications. The 94 GHz cavity-backed antennas based on a rectangular coaxial feeding network have been designed, fabricated, and tested. 13 dB gain for a 2 x 2 array, as well as antenna patterns are reported. In an eï¬€ort to facilitate high-accuracy measurement of the antenna array, an E-probe transition to waveguide and PolyStrata diode detectors were also designed and fabricated. AW-band rectangular coaxial passive monopulse comparator with integrated antenna array and diode detectors have also been presented. Measured monopulse nulls of 31.4 dB in the ΔAZ plane have been demonstrated. 94-GHz SiGe active monopulse comparator IC and 24 GHz RF-CMOS active monopulse comparator RFIC designs are presented, including detailed simulations of monopulse nulls and performance over frequency. Simulations of the W-band SiGe active monopulse comparator IC indicate potential for wideband operation, with 30 dB monopulse nulls from 75-105 GHz. For the 24-GHz active monopulse comparator IC, simulated monopulse nulls of 71 dB and 68 dB were reported for the azimuthal and elevational sweeps. Measurements of these ICs were unsuccessful due to layout errors and incomplete accounting for parasitics. Simulated results from a series of rectangular coaxial power corporate power combining structures have been presented, and their relative merits discussed. These designs include 2-1 and 4-1 reactive, Wilkinson, and Gysel combiners at V- and W-band. Measured back-to-back results from Gysel combiners at 60 GHz included insertion loss of 0.13 dB per division for a 2-1 combination, and an insertion loss of 0.3 dB and 0.14 dB for "planar" and "direct" 4-1 combinations, respectively. At 94 GHz, a measured insertion loss of 0.1 dB per division has been presented for a 2-1 Gysel combination, using a back-to-back structure. Preliminary designs for a solid-state power amplifier (SSPA) structure have also been presented. Finally, two conceptual monopulse transceivers will be presented, as a vehicle for integrating the various components demonstrated in this dissertation.
Active Antenna Bandwidth Control Using Reconfigurable Antenna Elements
Cummings, Nathan Patrick (Virginia Tech, 2003-12-08)
Reconfigurable antennas represent a recent innovation in antenna design that changes from classical fixed-form, fixed-function antennas to modifiable structures that can be adapted to fit the requirements of a time varying system. Advances in microwave semiconductor processing technologies have enabled the use of compact, ultra-high quality RF and microwave switches in novel aspects of antenna design. This dissertation introduces the concept of reconfigurable antenna bandwidth control and how advances in switch technology have made these designs realizable. Specifically, it details the development of three new antennas capable of reconfigurable bandwidth control. The newly developed antennas include the reconfigurable ring patch, the reconfigurable planar inverted-F and the reconfigurable parasitic folded dipole. The relevant background work to these designs is described and then design details along with computer simulations and measured experimental results are given.
Adaptive Arrays and Diversity Antenna Configurations for Handheld Wireless Communication Terminals
Dietrich, Carl B. (Virginia Tech, 2000-02-15)
This dissertation reports results of an investigation into the performance of adaptive beamforming and diversity combining using antenna arrays that can be mounted on handheld radios. Handheld arrays show great promise for improving the coverage, capacity, and power efficiency of wireless communication systems. Diversity experiments using a handheld antenna array testbed (HAAT) are reported here. These experiments indicate that signals received by the antennas in two-element handheld antenna arrays with spacing of 0.15 wavelength or greater can be combined to provide 7-9 dB diversity gain against fading at the 99% reliability level in non line-of-sight multipath channels. Thus, peer-to-peer systems of handheld transceivers that use antenna arrays can achieve reliability comparable to systems of single-antenna handheld units, with only one-fifth the transmitter power, resulting in lower overall power consumption and increased battery life. Similar gains were observed for spatial, polarization, and pattern diversity. Adaptive beamforming with single- and multi-polarized four-element arrays of closely spaced elements was investigated by experiment using the HAAT, and by computer simulation using a polarization-sensitive vector multipath propagation simulator developed for this purpose. Small and handheld adaptive arrays were shown to provide 25 to 40 dB or more of interference rejection in the presence of a single interferer in rural, suburban, and urban channels including line-of-sight and non line-of-sight cases. In multipath channels, these performance levels were achieved even when there was no separation between the transmitters in azimuth angle as seen from the receiver, and no difference in the orientations of the two transmitting antennas. This interference rejection capability potentially allows two separate spatial channels to coexist in the same time/frequency channel, doubling system capacity.
Analysis of a Helix Antenna Using a Moment Method Approach With Curved Basis and Testing Functions
Caswell, Eric D. (Virginia Tech, 1998-09-09)
Typically wire antenna structures are modeled by approximating curved structures with straight wire segments. The straight wire approximation yields accurate results, but often requires a large number of segments to adequately approximate the antenna geometry. The large number of straight wire segments or unknowns requires a large amount of memory and time to solve for the currents on the antenna. By using curved segments which exactly describe the contour of the antenna geometry the number of unknowns can be reduced, thus allowing for bigger problems to be solved accurately. This thesis focuses on the analysis of a helix antenna. The Method of Moments is used to solve for the currents on the antenna, and both the triangle basis and pulse testing functions exactly follow the contour of the helix antenna. The thin wire approximation is used throughout the analysis. The helix is assumed to be oriented along the z-axis with an optional perfect electric conductor (PEC) ground plane in the x-y plane. For simplicity, a delta gap source model is used. Straight feed wires may also be added to the helix, and are modeled similarly to the helix by the Method of Moments with triangular basis and pulse testing functions. The primary validation of the curved wire approach is through a comparison with MININEC and NEC of the convergence properties of the input impedance of the antenna versus the number of unknowns. The convergence tests show that significantly fewer unknowns are needed to accurately predict the input impedance of the helix, particularly for the normal mode helix. This approach is also useful in the analysis of the axial mode helix where the current changes significantly around one turn. Because of the varying current distribution, the improvement of impedance convergence with curved segments is not as significant for the axial mode helix. However, radiation pattern convergence improvement is found. Multiple feed structures for the axial mode helix are also investigated. In general, the many straight wire segments, and thus unknowns, that are needed to accurately approximate the current around one turn can be greatly reduced by the using the curved segment method.
Analysis of an Embedded Singularity Chipless RFID Tag
Blischak, Andrew Travis (Virginia Tech, 2011-06-30)
The objective of this research is to explore the feasibility of a chipless RFID tag that stores a data signature in the form of complex frequency plane singularities. To this end an existing chipless RFID tag, the notched elliptical dipole tag, was analyzed first with simulations and then measurements. A pole signature was extracted from simulations, and individual poles were determined via experimentation to be attributable to specific controllable features of the tag. The poles were shown to be independent of both excitation and observation. A prototype tag was measured, and the pole signature was retrieved from the scattered fields. The tag was successfully read for different orientations showing that embedded singularities can be used as a means for encoding and retrieving data.
Analysis of Highly Coupled Wideband Antenna Arrays Using Scattering Parameter Network Models
Takamizawa, Koichiro (Virginia Tech, 2002-12-10)
Wideband phased arrays require very tight element spacing to permit wide angle scanning of the main beam over the wide bandwidth. The consequence of tight spacing is very high mutual coupling among the elements in the array. Previous efforts by Virginia Tech Antenna Group has shown that the strong coupling can be utilized in arrays to obtain broadband frequency response while maintaining a small element spacing. However, mutual coupling between elements in a tightly coupled array can sometimes dramatically change the operating frequency, bandwidth, and radiation pattern from that of the single isolated element. Thus, there are some fundamental questions that remain regarding the effective operation of highly coupled arrays for beam forming, beam scanning, and aperture reconfiguration. Existing antenna pattern analysis techniques including the active element pattern method are inadequate for the application in highly coupled arrays. This dissertation focuses on the development of a new antenna array analysis technique. The presented method is based on the scattering parameter network descriptions of the array elements, associated feed network and the active element patterns. The developed model is general. It can be applied to an array of any size and configuration. The model can be utilized to determine directivity, gain and realized gain of arrays as well as their radiation efficiency and impedance mismatch. Using the network model, the relationship between radiation pattern characteristics and the input impedance characteristics of the array antennas becomes clear. Three types of source impedance matching conditions for array antennas are investigated using the model. A numerically simulated array of strip dipole array is used to investigate the effects of various impedance matching methods on the radiation pattern and impedance bandwidth. An application of network analysis is presented on an experimental investigation of $3\times 3$ Foursquare array test bed to further verify the concepts.
Analysis of infinite arrays of arbitrarily shaped planar radiating elements using a Floquet mode based Method of Moments approach
LaPean, James William (Virginia Tech, 1996-04-29)
Large phased array antennas are theoretically capable of delivering the directive gain of similarly sized aperture antennas while offering electronic beam scanning capabilities and greater operational flexibility. Unfortunately, the high cost associated with large phased antenna arrays has limited their use to highly specialized applications where no other antenna system configuration is possible. The recent development of less expensive microwave active devices has led to a renewed interest in large phased antenna arrays. These devices allow the amplification and signal processing required in phased antenna arrays to be distributed among many identical modules which combine the amplification, feed network, and radiating element sections of traditional antenna arrays. These modules can then be produced at a lower unit cost and result in an antenna system which is more easily integrated and repaired. The practicality of large phased antenna arrays is still limited by the great difficulty experienced in predicting their performance. Mutual coupling effects between the radiating elements produce significant variations from ideal array theory. The prediction and reduction of these effects requires a characterization approach which is computational rather than experimenta1. This document presents a new approach which allows the characterization of arbitrarily shaped planar radiating elements printed on a dielectric support slab backed by a perfect electric conductor ground plane. This analysis approach uses a Method of Moments technique to determine the electric current distribution over a set of bi-triangular sub-domain elements describing a single radiating element. The effects of mutual coupling in the fully active infinite antenna array are included in the analysis by a Floquet mode based Green's function used in the Method of Moments analysis. This characterization technique has been implemented in the computational electromagnetics code ASIA (Analysis Software for Infinite Arrays), The analysis approach presented here is validated by comparison with published input impedance data for two different radiating elements. Finally, preliminary analysis results are shown for a more complex radiating element.
Analysis of Periodic and Random Capacitively-Loaded Loop (CLL) Metamaterial Structures for Antenna Enhancement Applications
Hodge II, John Adams (Virginia Tech, 2014-07-02)
After being theorized by Veselago in 1967, recent developments in metamaterials over the last two decades have allowed scientists and researchers to physically demonstrate that artificial composite media can be engineered to exhibit exotic material properties, such as negative refractive index, by exploiting features in arrays of sub-wavelength unit inclusions. These unconventional electromagnetic properties are realized through the coupling of the microscopic unit inclusions, which govern the macroscopic properties of the structure. After demonstrating that a periodic array of capacitively-loaded loop (CLL) inclusions paired with continuous wire results in negative refraction, this study performs numerical simulations to characterize random metamaterial structures. These structures consist of CLLs that are randomized in both position and orientation. In addition, this thesis introduces an innovative antenna enhancing structure consisting of capacitively-loaded loop (CLL) metamaterial elements loaded radially around a standard dipole antenna at an electrically small distance. As a result of this innovative arrangement, the dipole antenna is easily transformed into a directive mechanically scanned antenna with high realized gain. The desired directivity and gain can be tuned based on the number of radial CLL fins placed around the dipole. Interactions between the antenna and metamaterial elements result in significant enhancement of the maximum radiated field amplitude and front-to-back ratio. This innovative CLL-loaded dipole antenna is compared to the conventional Yagi-Uda antenna. The structures presented in this thesis are modeled using full-wave simulation, and one antenna structure is experimentally verified as a proof-of-concept.
Analysis of phased array antenna radiation patterns including mutual coupling
Kelley, David Frederick (Virginia Tech, 1990-11-05)
Methods of expressing the radiation patterns of phased arrays in closed form that include the effects of radiated mutual coupling are investigated. The two basic methods considered are the classical array analysis method and the active element pattern methods. The theoretical derivations of the active element pattern methods are presented and the various types of active element patterns are defined. Also, a new method based on active element patterns, the hybrid active element pattern method, is introduced which accurately predicts the patterns of small and moderately-sized arrays of equally-spaced elements. Arrays of center-fed dipoles are considered in this study since dipole arrays can be fully characterized, including mutual coupling, using modem numerical electromagnetic analysis codes, thus allowing verification of the array analysis methods presented here. The results are general, however, and may be applied to arrays of any type of element. The array patterns computed using the classical analysis method and the active element pattern methods are compared to those computed using ideal array analysis and the highly-accurate numerical codes.
Analysis of Plasma Wave Irregularities Generated during Active Experiments in Near-Earth Space Environment
Bordikar, Maitrayee Ranade (Virginia Tech, 2013-05-26)
This work focuses on the analysis of plasma irregularities generated during two active space experiments: the injection of an artificial dust layer, and high-power radio waves. The objective of the "first experiment is to examine the effects of artificially created dust layers on the scatter of radars from plasma irregularities embedded in dusty plasma in space. This is an alternate approach for understanding the mechanisms of enhanced radar scatter from plasma irregularities embedded in Noctilucent Clouds and Polar Mesospheric Summer Echoes. The second experiment involves a transmission of high power electromagnetic waves into the ionospheric plasma from the ground, which can excite stimulated electromagnetic emissions offset from the transmitter frequency. These stimulated electromagnetic emissions provide diagnostic information of the ionosphere and thus can be used to investigate fundamental physical principles which govern the earth\'s ionosphere, so that present and future transmission technologies may take into account the complexities of the ionosphere. The interaction altitude of the artificial dust layer and high power radio waves is approximately 250 km and 160 km respectively, thus dealing with uniquely different regions of the ionosphere. Each experiment is discussed separately using theoretical, observational and advanced computational methodologies. The study first investigates plasma turbulence associated with the creation of an artificial dust layer in the earth's ionosphere. Two scenarios are considered for plasma irregularity generation as dust is injected at an oblique angle across the geomagnetic field. The first is a shear-driven plasma instability due to inhomogeneities in the boundary layer between the injected charged dust layer and the background plasma. This begins to appear at very early times once the dust is released into the space plasma, which is of the order or less than the dust charging time period. The second mechanism is free streaming of the charged dust relative to the background plasma. This produces irregularities at times much longer than the dust charging period and also longer than the dust plasma period. Although both mechanisms are shown to produce turbulence in the lower hybrid frequency range, the resulting irregularities have important differences in their physical characteristics. A comparison between the processes is made to determine the consequences for upcoming observations. Both processes are shown to have the possibility of generating turbulence after the release of dust for the regimes of upcoming space experiments over a range of timescales. This work also presents the first observations of unique narrowband emissions ordered near the Hydrogen ion (H+) gyro-frequency (fcH) in the Stimulated Electromagnetic Emission (SEE) spectrum when the transmitter is tuned near the second electron gyro-harmonic frequency (2fce), during ionospheric modification experiments. The frequency structuring of these newly discovered emission lines is quite unexpected since H+ is known to be a minor constituent in the interaction region which is near 160 km altitude. The spectral lines are typically shifted from the pump wave frequency by harmonics of a frequency about 10% less than fcH (" 800 Hz) and have a bandwidth of less than 50 Hz which is near the O+ gyro-frequency fcO. A theory is proposed to explain these emissions in terms of a Parametric Decay Instability (PDI) in a multi-ion species plasma due to possible proton precipitation associated with the disturbed conditions during the heating experiment. The observations can be explained by including several percent H+ ions into the background plasma. The implications are new possibilities for characterizing proton precipitation events during ionospheric heating experiments.
Analysis of the Radiation Mechanisms in and Design of Tightly-Coupled Antenna Arrays
Vogler, Terry Richard (Virginia Tech, 2010-09-10)
The objective of this research is to design well-tuned, wideband elements for thin planar or cylindrically conformal arrays of balanced elements fed over ground. These arrays have closely spaced elements to achieve wide bandwidths through mutual coupling. This dissertation develops two wideband designs in infinite, semi-infinite, and finite array configurations. The infinite array is best for element tuning. This research advances a concept of a distributed, parallel capacitance between elements and across feeds that must be mutually altered for tuning. Semi-infinite techniques limit the problem space and determine the proper resistive loads to control the low-frequency array-guided surface wave (AGSW). The tight physical placement also forms a periodic structure that, along with the array boundary, launches a wave across the array surface. Options to suppress this surface wave are resistive loading and cylindrical conformations. AGSW control is necessary to achieve a maximum bandwidth, but lower radiation or aperture efficiency results. Conformation is shown to be an ineffective method for AGSW control alone. The Wrapped Bowtie design emerges as a novel design offering nearly a 10:1 bandwidth as a finite array over ground. Some bandwidth comes from the losses in radiation efficiency, which is necessary to control the AGSW; however, its simulated VSWR < 3 bandwidth in an infinite array is 7.24:1 with full efficiency. Less than perfect efficiency is required to mitigate surface wave effects, unless bandwidth is to be compromised. That loss may be as radiation or aperture efficiency, but it is unavoidable if the infinite array bandwidth is to be maintained in finite array designs. Lastly, this research articulates a development path for tightly-coupled arrays that extends in stages from infinite to semi-infinite, and thence finite layouts. Distinctions are explained and defended for the design focus at each stage. Element design, tuning, and initial feed design occur at the infinite array stage; AGSW suppression occurs at the semi-infinite stage; and design confirmation occurs only with the finite array.
Analysis, design, and fabrication of Archimedean spiral antennas
Werntz, Paul C. (Virginia Tech, 1988-08-05)
The uses for wideband antennas include wideband measurement systems, spread spectrum communications systems, feeds for reflectors and elements in wideband arrays. Here, wideband antennas are discussed and Archimedean spiral antennas are found to be appropriate elements for use in a new type of wideband array referred to as the wideband switched array. The design of an Archimedean spiral and a necessary wideband balun transformer feed structure are presented. To aid in the design, the Electromagnetic Surface Patch Code (ESP) developed by Ohio State University is used. The spiral and feed structure are constructed and measured results are compared to predictions obtained by ESP.
Antenna effects on indoor wireless channels and a deterministic wide-band propagation model for in-building personal communication systems
Ho, Chung-Man Peter (Virginia Tech, 1993-04-22)
While the application of antenna diversity in a narrow band communication system is well understood, little research has been done on antenna effects in wide-band channels. Research has shown that circular polarization (CP) is more robust in combating multipath than linear polarization in line-of-sight channels. One objective of this thesis is to study the effects of antenna polarization and antenna pattern on multipath delay spread and path loss in indoor obstructed (OBS) wireless channels. A wide-band experiment was performed in a two-floored modern office building at 2.4SGHz in August 1991. Some preliminary results are as follows. Circular polarization cannot reduce delay spread in OBS channels and CP signals are more vulnerable to depolarization in OBS channels. Our results show that vertically polarized (VP) directional antennas at both the transmitter and the receiver can give better delay spread and path loss results than other antenna combinations. The performance of VP directional antennas are found to be sensitive to the alignment of the antennas, and the performance gain over omnidirectional antennas degrades as shadowing effects increase. In the second half of the thesis, a deterministic wide-band propagation model that can predict channel impulse responses inside buildings is proposed and implemented. The three dimensional image-based propagation model includes effects of antenna pattern, antenna polarization, geometry of the building, and building materials. Comparisons between measured and predicted power delay profiles are given in Chapter Seven. Preliminary results show that the worst case path loss error is IOdB, and the standard deviation of path loss error is 4.6dB. For most cases, predicted rms delay spread values are 20ns within the measured values. Possible prediction errors are due to unmodeled furniture inside the offices and limitations of Geometrical Optics (GO) assumptions. The algorithm is shown to be more efficient than brute force ray tracing algorithm if the number of objects are on the order of a few hundred. Acceleration techniques for the algorithm are also discussed in the thesis.
Applications in Remote Sensing Using the Method of Ordered Multiple Interactions
Westin, Benjamin Alexander (Virginia Tech, 2013-04-24)
The Method of Ordered Multiple Interactions provides a numerical solution to the integral
equations describing surface scattering which is both computationally efficient and reliably
convergent. The method has been applied in a variety of ways to solving the electromagnetic
scattering from perfectly-conducting rough surfaces. A desire to more accurately predict
the scattering from natural terrain has led to the representation of the surface material as
penetrable instead of conductive.

For this purpose, the Method of Ordered Multiple Interactions is applied to numerically
solve the electromagnetic scattering from randomly-rough dielectric surfaces. A primary
consequence of the penetrable surface material is the introduction of a pair of coupled integral equations in place of the single integral equation used to solve the problem with a perfectly conducting surface. The method is tested and analyzed by developing independent scattering solutions for canonical cases in a transform domain and by comparing results with solutions from other techniques.

The dielectric implementation of the Method of Ordered Multiple Interactions is used to solve
the electromagnetic scattering from a class of randomly-rough dielectric surfaces. This allows
for the characterization of the effect of a number of transmitter and surface parameters in the
scattering problem, observing bistatically and also specifically in the backscatter direction.

MOMI is then applied as a method to examine subsurface penetration characteristics from
a similar family of rough surfaces. Characteristics of the environment parameters and the
scattered field itself are examined, and the numerical challenges associated with observing
beneath the surface are identified and addressed.

The Method of Ordered Multiple Interactions is then incorporated as a major component of
a larger solution which computes the total scattering when a dielectric object is buried just
beneath the rough surface. This hyrid approach uses MOMI and the Method of Moments to
iteratively account for multiple interactions between the target and the dielectric interface,
enabling the study of scattering from the combined environment of a rough surface and the
embedded object, as well as the individual scattering events which combine to form the
steady-state solution.
Blind Identification of MIMO Systems: Signal Modulation and Channel Estimation
Dietze, Kai (Virginia Tech, 2005-08-25)
Present trends in communication links between devices have opted for wireless instead of wired solutions. As a consequence, unlicensed bands have seen a rise in the interference level as more and more devices are introduced into the market place that take advantage of these free bands for their communication needs. Under these conditions, the receiver's ability to recognize and identify the presence of interference becomes increasingly important. In order for the receiver to make an optimal decision on the signal-of-interest, it has to be aware of the type (modulation) of interference as well as how the received signals are affected (channel) by these impediments in order to appropriately mitigate them. This dissertation addresses the blind (unaided) identification of the signal modulations and the channel in a Multiple Input Multiple Output (MIMO) system. The method presented herein takes advantage of the modulation induced periodicities of the signals in the system and uses higher-order cyclostationary statistics to extract the signal and channel unknowns. This method can be used to identify more signals in the system than antenna elements at the receiver (overloaded case). This dissertation presents a system theoretic analysis of the problem as well as describes the development of an algorithm that can be used in the identification of the channel and the modulation of the signals in the system. Linear and non-linear receivers are examined at the beginning of the manuscript in order to review the a priori information that is needed for each receiver configuration to function properly.
A Class of Robust and Efficient Iterative Methods for Wave Scattering Problems
Adams, Robert John (Virginia Tech, 1998-12-17)
Significant effort has recently been directed towards the development of numerically efficient iterative techniques for the solution of boundary integral equation formulations of time harmonic scattering problems. The primary result of this effort has been the development of several advanced numerical techniques which enable the dense matrix-vector products associated with the iterative solution of boundary integral equations to be rapidly computed. However, an important aspect of this problem which has yet to be adequately addressed is the development of rapidly convergent iterative techniques to complement the relatively more mature numerical algorithms which expedite the matrix-vector product operation. To this end, a class of efficient iterative methods for boundary integral equation formulations of two-dimensional scattering problems is presented. This development is based on an attempt to approximately factor (i.e., renormalize) the boundary integral formulation of an arbitrary scattering problem into a product of one-way wave operators and a corresponding coupling operator which accounts for the interactions between oppositely propagating waves on the surface of the scatterer. The original boundary integral formulation of the scattering problem defines the coupling between individual equivalent sources on the surface of the scatterer. The renormalized version of this equation defines the coupling between the forward and backward propagating fields obtained by re-summing the individual equivalent sources present in the original boundary integral formulation of the scattering problem. An important feature of this class of rapidly convergent iterative techniques is that they are based on an attempt to incorporate the important physical aspects of the scattering problem into the iterative procedure. This leads to rapidly convergent iterative series for a number of two-dimensional scattering problems. The iterative series obtained using this renormalization procedure are much more rapidly convergent than the series obtained using Krylov subspace techniques. In fact, for several of the geometries considered the number of iterations required to achieve a specified residual error is independent of the size of the scatterer. This desirable property of the iterative methods presented here is not shared by other iterative schemes for wave scattering problems. Moreover, because the approach used to develop these iterative series depends only on the assumption that the total field can be approximately represented by a summation of independent and oppositely directed waves (and not on the presence of special geometries, etc.), the proposed iterative methods are very general and are thus applicable to a large number of complex scattering problems.
Commodore Perry's 1853 Japanese Expedition: How Whaling Influenced the Event that Revolutionized Japan
Burcin, Terry (Virginia Tech, 2005-05-05)
In July 1853, Commodore Matthew C. Perry illegally entered Tokyo Bay with a fleet of four American warships and challenged Japan's isolationist position towards the United States. This radical diplomatic effort concluded with a return voyage a year later and ended Japan's self-imposed isolation from the Western world. Historians, in an attempt to explain the motivations behind Perry's voyage, cite an American commercial desire for Chinese trade as the main reason behind the Japanese Expedition's dispatch. This historical perspective ignores the important economic and political influence the whaling industry played in spurring American politicians to confront Japanese isolationism. It is incorrect to assert that whaling, and not America's desire to gain access to China, was the main reason behind the 1853 Japanese Expedition. This paper's objective is to understand how whaling influenced Perry's mission. It should be read as a supplement to current historical scholarship concerning America's decision to send a naval force into Japanese waters.
A Comprehensive Investigation of New Planar Wideband Antennas
Suh, Seong-Youp (Virginia Tech, 2002-07-29)
Broadband wireless communications require wideband antennas to support large number of users and higher data rates. Desirable features of a wideband antenna are low-profile, dual-polarization and wide bandwidth in a compact size. Many existing wideband antennas are large in size and some have only circular polarization. On the other hand low-profile, dual-polarized antennas frequently have limited bandwidth. This dissertation reports on results from original research into several new wideband antennas. All are compact and planar, and many are low-profile and dual-polarized. Since 1994, Virginia Tech Antenna Group (VTAG) has performed research on the wideband, low-profile and dual-polarized antennas of compact size. This research resulted in the following antenna innovations: the Fourpoint, Fourtear, PICA (Planar Inverted Cone Antenna), diPICA (dipole PICA) and LPdiPICA (Low-Profile diPICA) antennas. They are all planar in geometry so one can easily construct them in a compact size. The antennas were characterized and investigated with extensive simulations and measurements. The computed and measured data demonstrates that some of the antennas appear to have the characteristics of the self-complementary antenna and most of the proposed antennas provide more than a 10:1 impedance bandwidth for a VSWR < 2. Patterns, however, are degraded at the high end of the frequency. Several tapered ground planes were proposed to improve the radiation pattern characteristics without degrading the impedance performance. A simulation result proposed a possibility of another antenna inventions providing 10:1 pattern bandwidth with the 10:1 impedance bandwidth. Research into wideband antennas demonstrated that the newly invented antennas are closely related each other and are evolved from a primitive element, PICA. Not only the comprehensive investigation but also a practical antenna design has been done for commercial base-station array antennas and to phased array antennas for government applications. This dissertation presents results of comprehensive investigation of new planar wideband antennas and its usefulness to the broadband wireless communications.
Control of thermal runaway in microwave resonant cavities
Wu, X.; Thomas, J. R.; Davis, William A. (American Institute of Physics, 2002-09-15)
This article reports direct experimental evidence of the so-called "S curve" of temperature versus electrical field strength when materials with positive temperature dependence of dielectric loss are heated in a microwave resonant cavity applicator. A complete discussion of how the experimental results were achieved is presented. From the experimental results, we believe the S curve theory provides an incomplete explanation of thermal runaway in microwave heating. To understand microwave heating in a resonant cavity, cavity effects must be considered. To explain the experimental results, a theoretical model based on single-mode waveguide theory is developed. Finally, a method to control thermal runaway is described. (C) 2002 American Institute of Physics.
A Coupled Heat Transfer and Electromagnetic Model for Simulating Microwave Heating of Thin Dielectric Materials in a Resonant Cavity
McConnell, Brian Gregory (Virginia Tech, 1999-12-09)
Microwave heating is an emerging but still underutilized tool in modern industrial applications. The task of designing microwave applicators for heating industrial materials with temperature-dependent properties is challenging, and trial-and-error system prototyping is an expensive and wasteful means to accomplish this goal. The purpose of this work is to combine existing heat transfer and electromagnetic models to provide a complete simulation for heating dielectric materials in a resonant microwave cavity. The numerical simulation is validated by comparison to several independent sets of experimental data. The ultimate goal is to provide a research tool that will facilitate the industrial microwave applicator design process. With a complete, accurate, and user-friendly numerical simulation, parameters affecting the temperature distribution in stationary and moving process materials can be studied to optimize the results before the first prototype is made. This work also explores the sources of power loss in a microwave system and develops means for quantifying these power losses.

Browsing by Author "Davis, William A."

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