Browsing by Author "Davis, Bradley A."

Now showing 1 - 11 of 11
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    Adaptive Pattern Modeling for Large Reflector Antennas
    Sengupta, Ramonika (Virginia Tech, 2022-08-04)
    This thesis presents methods for modeling the pattern of large axisymmetric paraboloidal focus-fed reflector antenna systems. The intended application of these methods is to improve the performance of time-domain interference canceling (TDC) in radio astronomy. The first method yields a closed-form expression for the antenna pattern with parameters accounting for the focal ratio and feed pattern. In subsequent adaptive methods, parameters of this model are calculated using measurements of interference signals. The corrected pattern model improves the prediction of the change in the true pattern for future times. The methods are compared by (1) comparing the error in the pattern model with respect to the true pattern and (2) comparing the pattern value update period required to achieve a specified level of residual interference when used in TDC. The efficacy of the pattern modeling methods is demonstrated by showing that the error in the pattern model decreases and the pattern value needs to be updated at a much slower rate for effective TDC.
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    Characterization of the Integration of Additively Manufactured All-Aromatic Polyimide and Conductive Direct-Write Silver Inks
    Oja, Thomas Edward (Virginia Tech, 2020-12-07)
    Hybridizing additive manufacturing (AM) structures and direct write (DW) deposition of conductive traces enables the design and physical creation of integrated, complex, and conformal electronics such as embedded electronics and complex routing on a fully AM structure. Although this hybridization has a promising outlook, there are several key AM substrate-related limitations that limit the final performance of these hybridized AM-DW electronic parts. These limitations include low-temperature processability (leading to high trace resistivity) and poor surface finish (leading to electronic shorts and disconnections). Recently discovered ultraviolet-assisted direct ink write (UV-DIW) all-aromatic polyimide (PI) provides an opportunity to address these previous shortcomings previously due to its high-temperature stability (450C) and superior surface finish (relative to other AM processes). The primary goal of this thesis is to characterize the integration of this UV-DIW PI with DW-printed conductive inks as a means for obtaining high-performance hybrid AM-DW electronics. This goal has been achieved through an investigation into the increased temperature stability of AM PI on the conductivity and adhesion of DW extrusion and aerosol jet (AJ) silver inks, determining the dielectric constant and dissipation factor of processed UV-DIW PI, and determining the achievable microwave application performance of UV-DIW PI. These performance measurements are compared to commercially-available PI film and relative to existing AM substrates, such as ULTEM 1010. The temperature stability of UV-DIW PI enabled higher-temperature post-processing for the printed silver traces, which decreased DIW trace resistivity from 14.94±0.55 times the value of bulk silver at 160 °C to 2.16±0.028 times the resistivity of bulk silver at 375 °C, and AJ silver trace resistivity from 5.27±0.013 times the resistivity of bulk silver at 200 °C to 1.95±0.15 times the resistivity of bulk silver at 350 °C. The adhesion of these traces was not negatively affected by higher processing temperatures, and the traces performed similarly on UV-DIW PI and commercial PI. Furthermore, at similar thicknesses, UV-DIW PI was found to have a similar dielectric constant and dissipation factor to commercial Dupont Kapton PI film from 1 kHz to 1 MHz, indicating its ability to perform highly as a dielectric electronics substrate. Finally, the decrease in resistivity was able to decrease the gap in microwave stripline transmission line performance when compared with ULTEM 1010 processed at 200°C, with peak 10 GHz S21 loss differences decreasing from 2.46 dB to 1.32 dB after increasing the UV-DIW processing temperature from 200 °C to 400°C.
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    A clutter power computation procedure for airborne Doppler radar
    Davis, Bradley A. (Virginia Polytechnic Institute and State University, 1988)
    This report presents a general procedure for estimating the average clutter power received by an airborne Doppler radar as a function of time and frequency. Its purpose is to construct the received power waveform at the output of a matched filter for use in the development of clutter maps related to Doppler signal processing. In developing the algorithm, several of its features are given close examination; this includes a numerical and analytical justification for the azimuth to Doppler coordinate transformation, and the power spectrum returned to the airborne Doppler radar from the surface it illuminates. This algorithm produces a continuous power returned waveform versus time. Hence, this algorithm has the flexibility to accept any surface radar cross section, antenna gain, and pulse type (a matched filter receiver is assumed) as a function of range and azimuth. It was discovered that the algorithm could be executed with significantly less computer time if the integration surface area was reduced by exploiting some physical insight. In addition, it was discovered through example that the refracting effects of the Earth's atmosphere become important as the radar's beam approaches the horizon. Finally, return power waveforms are studied for two different situations: near nadir pointing, and near horizon pointing. The manner in which the matched filter ambiguity function and the spectrum of backscattered power combine to produce these waveforms is examined.
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    Fabricating Multifunctional Composites via Transfer of Printed Electronics Using Additively Manufactured Sacrificial Tooling
    Viar, Jacob Zachary (Virginia Tech, 2022-06-07)
    Multifunctional composites have gained significant interest as they enable the integration of sensing and communication capabilities into structural, lightweight composites. Researchers have explored additive manufacturing processes for creating these structures through selective patterning of electrically conductive materials onto composites. Thus far, multifunctional composite performance has been limited by the conductivity of functional materials used, and the methods of integration have resulted in compromises to both structural and functional performance. Integration methods have also imposed limitations on part geometry due to an inability to adequately deposit conductive material over concave surfaces. Proposed methods of integrating functional devices within composites have been shown to negatively affect their mechanical performance. This work presents a novel method for integrating printed electronics onto the interior surfaces of closed, complex continuous fiber composite structures via the transfer of selectively printed conductive inks from additively manufactured sacrificial tooling to the composite surface. The process is demonstrated by creating multifunctional composites via embossing printed electronics onto structural composites without negatively affecting the mechanical performance of the structure. Additionally, this process expands the ability to pattern devices onto complex surfaces and demonstrates that the transferred functionality is well integrated (adhered) with the composite surface. The process is further validated through the successful completion of two separate case studies. The first is the integration of a functioning strain gauge onto an S-glass/epoxy composite, while a second process demonstration shows a composite surface featuring a band stop filter at the X-band, otherwise known as a frequency selective surface (FSS), to show the process' suitability for high performance, aerospace grade multifunctional composites.
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    Magnetic Targeted Drug Delivery
    Leach, Jeffrey Harold (Virginia Tech, 2003-02-12)
    Methods of guiding magnetic particles in a controlled fashion through the arterial system in vivo using external magnetic fields are explored. Included are discussions of applications, magnetic field properties needed to allow guiding based on particle characteristics, hemodynamic forces, the uniformity of field and gradients, variable tissue characteristics, and imaging techniques employed to view these particles while in transport. These factors influence the type of magnetic guidance system that is needed for an effective drug delivery system. This thesis reviews past magnetic drug delivery work, variables, and concepts that needed to be understood for the development of an in vivo magnetic drug delivery system. The results of this thesis are the concise study and review of present methods for guided magnetic particles, aggregate theoretical work to allow proper hypotheses and extrapolations to be made, and experimental applications of these hypotheses to a working magnetic guidance system. The design and characterization of a magnetic guidance system was discussed and built. The restraint for this system that balanced multiple competing variables was primarily an active volume of 0.64 cm3, a workspace clearance of at least an inch on every side, a field of 0.3T, and a local axial gradient of 13 T/m. 3D electromagnetic finite element analysis modeling was performed and compared with experimental results. Drug delivery vehicles, a series of magnetic seeds, were successfully characterized using a vibrating sample magnetometer. Next, the magnetic seed was investigated under various flow conditions in vitro to analyze the effectiveness of the drug delivery system. Finally, the drug delivery system was successfully demonstrated under limiting assumptions of a specific magnetic field and gradient, seed material, a low fluid flow, and a small volume.
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    The Polarimetric Impulse Response and Convolutional Model for the Remote Sensing of Layered Vegetation
    Kramer, Tyler Christian (Virginia Tech, 2007-03-05)
    To date, there exists no complete, computationally efficient, physics-based model to compute the radar backscatter from forest canopies. Several models attempt to predict the backscatter coefficient for random forest canopies by using the Vector Radiative Transfer (VRT) Theory with success, however, these models often rely on purely time-harmonic formulations and approximations to integrals. Forms of VRT models have recently been developed which account for a Gaussian pulse incident waveform, however, these models often rely heavily on very specific and obfuscated approximations to solve the associated integrals. This thesis attempts to resolve this problem by outlining a method by which existing, proven, time harmonic solutions to the VRT equation can be modified to account for arbitrary pulse waveforms through simple path delay method. These techniques lend physical insight into the actual scattering mechanisms behind the returned waveform, as well as offer explanations for why approximations of previous authors' break down in certain regions. Furthermore, these radiative transfer solutions can be reformulated into a convolutional model which is capable of quickly and accurately predicting the radar return of random volumes. A brief overview of radiative transfer theory as it applies to remote sensing is also given.
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    Propagation and Scattering of Waves by Terrain Features
    Davis, Bradley A. (Virginia Tech, 2000-06-13)
    The intent of this dissertation is to obtain estimates of the effects of natural terrain features on the propagation and scattering of waves. It begins with the rough knife obstruction case, moves into rough surfaces and finally concludes with several approaches to a foliage covered rough surface. Each of these problems is encountered in radar, remote sensing and communication systems. The first topic in this dissertation is the study of the effect of random edge roughness on the diffraction of a wave. This has been accomplished by approximating the field beyond the diffracting half-plane through the use of spectral techniques and the Kirchhoff approximation. The relationships developed for the mean or average diffracted field and the incoherent diffracted power are studied for a range of electrophysical parameters that are representative of the situation encountered in a point-to-point communications link with blockage by a rough edged half plane. The interpretation of the results is facilitated by the observation that the total diffracted field is a superposition of the incident field and the edge-diffracted field. When the roughness on the edge increases, the edge diffracted-field becomes more incoherent and the phase interference consequently diminishes, leading to an attenuation of the oscillations in the coherent or mean total field. The model also addresses the effects of the knife edge in directions off the line-of-sight path as well as its effects on pulse propagation. Next, rough surface scattering effects are addressed. Extending the idea of the knife edge diffraction, this dissertation builds on the topic of a wedge on a plane by adapting the Method of Multiple Ordered Interactions (MOMI) to the dielectric surface. In this development, the coupled integral equations governing the scattering by a dielectric surface are combined into a single equation wherein the lossy dielectric enters the solution as a perturbation of the result for a perfectly conducting surface. Hence, the solution is not only exact, but as the loss increases, the convergence is rapid. Next, the Kirchhoff approximation is expanded to a two-frequency form for use with the later chapters which deal with pulse scattering by rough surfaces. Example waveform calculations are given. Propagation and scattering by a volume of scatterers over a surface is then examined. Starting from the radiative transfer equations, a model is developed herein for scattering from vegetated rough terrain. It assumes completely incoherent scattering and includes contributions from both the vegetation and the surface scattering along with a relatively simple accounting for their interaction. The model is developed into a form that easily separates the three primary components of the scattering problem - the radar system, the geometry, and the environment, and then recombines them through a multiple convolution. Extending the basic model to volumes for which multiple scattering is important is accomplished through the use of effective parameters. These effective parameters are obtained by comparing the model with pulsed radar data at normal incidence, i.e., looking directly down through the foliage and onto the ground. Hence, our overall model is a hybrid approach wherein the basic physics are retained in the simple solution. It is then extended to a more complicated environment through the use of these effective parameters. Example waveform calculations are given. The simple model assumes that multiple interactions are insignificant and that only narrow-band signals and narrow-beamwidth antenna patterns are used. Consequently, a more general radiative transfer approach is applied to the propagation of a beam through the random medium. This effort is a test of the narrow beamwidth and forward-backward scattering approximations implicit in the convolutional model. Next, the same convolutional model is developed using wave theory in order to clarify the assumptions and lend some physical meaning to the free parameters of the convolutional model. First the single scatter theory, with strictly forward and backward scattering is shown to be equivalent to the convolutional approach derived with radiative transfer theory. Next, multiple scattering in a discrete random medium is investigated in the "extended" Twersky approximation [Tsolakis, 1985]. This development leads to the mean Green's function for the medium, a form of the Distorted Wave Born Approximation and to a two-frequency radiative transfer equation. This transfer equation is then shown to simplify under the forward-backward assumption, eventually leading to a form which is compatible with the convolutional result. Finally, the effects of multiple scattering between the volume and its boundary, the rough surface, have been investigated. Using a numerical implementation (MOMI) of a scatterer over a rough surface, the orders of significant multiple interactions between the rough surface and the volume scattering components have been simulated. It is demonstrated that foliage components well above the rough surface may be treated as non-interacting; this includes components other than the trunks, which were not simulated. However, it is evident that multiple scattering effects may be significant for large objects near the rough surface. This work has been supported by grants from the Bradley Department of Electrical and Computer Engineering, National Science Foundation, and the Virginia Space Grant Consortium. Additional support has been provided by the U.S. Air Force at Hansom Airforce Base under grant F19628-96-C-0071, and U.S. Army Research Office under grant DAAG55-97-1-0164.
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    Rim-Mounted Reconfigurable Surfaces for Interference Nulling in Focus-Fed Axisymmetric Reflector Antenna Systems
    Yip, Alec Johnathan (Virginia Tech, 2024-08-20)
    This thesis addresses the use of an electronic reconfigurable surface (ERS) attached to an axisymmetric paraboloidal reflector antenna to mitigate interference through nulling for radio astronomy applications. The ERS is a reflectarray that consists of multiple elements, which can be electronically controlled to modify the reflected field. Previous research focused on a "conformal" system with elements placed on a parabololoidal surface extending beyond the unmodified reflector. These studies concluded that nulling could be achieved without compromising the gain and shape of the main lobe. This thesis builds on previous research by addressing two topics. The first topic is minimization of the ERS rim width. A closed form expression for the minimum width required for effective nulling in the close-in sidelobes is derived in this thesis. The second topic addressed in this thesis is ERS configurations comprised of flat panels as an alternative to the conformal ERS implementation. Flat panels may be easier to manufacture compared to their conformal counterpart. Flat panel ERS implementations are evaluated, and it is concluded that they are a suitable alternative to conformal ERS implementation.
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    Selective Deposition of Copper Traces onto Additively Manufactured All-Aromatic Polyimides via Laser Induced Graphene to Enable Conformal Printed Electronics
    Wotton, Heather Dawn (Virginia Tech, 2024-04-03)
    The hybridization of direct write (DW) and additive manufacturing (AM) technologies to create additively manufactured electronics (AME) has enabled the integration of electrical functionality to form multifunctional AM components. Current work in AME has demonstrated the integration of conductive traces into and onto geometries and form factors that are not possible through traditional electronics packaging processes. This has largely been accomplished by using AM and DW technology to deposit conductive inks to form interconnects on the surface of AM substrates or within multimaterial AM geometries. However, the requisite thermal post-processing and high resistivity of the conductive inks and the limitations in thermal and dielectric performance of printable substrates commonly used in AME restrict the capabilities of these parts. This thesis proposes an alternative process for the conformal deposition of low resistivity traces on additively manufactured all-aromatic polyimides (AM-PI) without the use of conductive inks. This is accomplished through the selective patterning of laser induced graphene (LIG), a porous 3D graphene fabricated via laser irradiation, onto the AM-PI. While the resultant LIG is conductive, its resistivity is further reduced by the electrodeposition of copper (Cu-LIG). In this thesis, the synthesis of LIG on AM-PI, thermally post processed to 240℃, 300℃, and 450℃, is demonstrated and characterized through sheet resistance measurements and Raman spectroscopy. AM-PI post-processed to 300℃ demonstrated the lowest resistivity LIG formation (13.8 Ω/square). The resistivity of Cu-LIG is compared to an industry standard silver ink (Micromax CB028) used in direct write hybrid manufacturing applications. Cu-LIG was found to have a measured resistivity (1.39e-7 Ω·m), two orders of magnitude lower than the measured resistivity of the CB028 silver ink (1.62e-5 Ω·m). Additionally, the current capacity of the Cu-LIG was demonstrated and Joule heating of the material was observed via IR thermography. Cu-LIG demonstrated no failure of conductive trace or substrate under 5A of current for 2 minutes, heating to a maximum recorded temperature of 76.3℃. Several multifunctional components were fabricated as case studies to further validate the process. Several small passive electronic devices (e.g., a heater and an interdigitated capacitor) are fabricated to demonstrate selective deposition of complex copper traces. The fabrication of an Archimedes spiral on a hemispherical substrate via Cu-LIG is completed to demonstrate the ability to use the process to fabricate conformal conductive traces. An LED circuit is fabricated on a face-center cubic AM-PI lattice which demonstrates multi-planar fabrication on geometrically complex 3D printed substrates.
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    Switching Stage Design and Implementation for an Efficient Three-Phase 5kW PWM DC-DC Converter
    Urciuoli, Damian (Virginia Tech, 2003-08-04)
    With the development of fuel cell based power systems, the need for more advanced DC-DC power converters has become apparent. In such applications DC-DC converters provide an important link between low voltage fuel cell sources and inverter buses operating at significantly higher voltages. Advancements in converter efficiency, cost reduction, and size reduction are the most necessary. These challenges are formidable, even when considering the improvements made to conventional DC-DC topologies. However, it can be possible to achieve these criteria through the implementation of more advanced topologies. A recently developed efficient three-phase DC-DC topology offers benefits over standard designs. Passive component sizes and output ripple voltage were reduced as a result of an effective boost in switching frequency. Converter output voltage was reached more easily due to an increased transformer voltage boost ratio in addition to the turns ratio. For cost reduction, the converter was designed and built with discrete components instead of more expensive integrated modules. This thesis presents an overview of the three-phase converter, with a detailed focus on the design, implementation, and performance of the switching stage. The functionality of the three-phase topology is covered along with the selection of converter components. Simulation results are shown for both ideal and real converter models. Considerations for the switching device package with respect to circuit board and heat sinking configurations are discussed in support of the selection of an insulated metal substrate (IMS) circuit board. An effective circuit layout designed to minimize parasitic trace inductances as well as provide favorable component positioning is presented. Experimental converter test results are shown and the causes of undesired effects are identified. Switching stage modifications and their results are discussed along with the benefits of proposed future design enhancements.
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    Tilt-Compensated Magnetic Field Sensor
    Bingaman, Adam Neal (Virginia Tech, 2010-05-13)
    Motion and tilt have long hindered the accuracy, reliability, and response of magnetic detection systems. Perturbations in the magnetic field reading resulting from motion cause degradation of the output signal, compromising the performance and reliability of the magnetometer system. The purpose of this document is to describe the development, construction, and testing of a tilt-stabilized three-axis magnetic field sensor. The sensor is implemented as a three-axis general-purpose magnetic field sensor, with the additional capability of being implemented as a compass. Design and construction of system hardware is discussed, along with software development and implementation. Finite impulse response filters are designed and implemented in hardware to filter the acquired magnetic signals. Various designs of median filters are simulated and tested for smoothing inclination signal irregularities and noise. Trigonometric conversions necessary for tilt-compensation are calculated in software using traditional methods, as well as the Coordinate Rotation Digital Computer (CORDIC) algorithm. Both calculation methods are compared for execution time and efficiency. Successful incorporation of all design aspects leads to detection and output of stable earth magnetic fields, sinusoidal signals, and aperiodic signatures while the magnetometer system is subject to significant tilt motion. Optimized system execution time leads to a maximum detectable signal bandwidth of 410 Hz. Integration of azimuth angle calculation is incorporated and is successfully tested with minimal error, allowing the system to be used as a compass. Results of the compensated system tests are compared to non-compensated results to display system performance, including tilt-compensation effectiveness, noise attenuation, and operational speed.