Matching Items (22)

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Magneto-dielectric wire antennas theory and design

Description

There is a pervasive need in the defense industry for conformal, low-profile, efficient and broadband (HF-UHF) antennas. Broadband capabilities enable shared aperture multi-function radiators, while conformal antenna profiles minimize physical damage in army applications, reduce drag and weight penalties in

There is a pervasive need in the defense industry for conformal, low-profile, efficient and broadband (HF-UHF) antennas. Broadband capabilities enable shared aperture multi-function radiators, while conformal antenna profiles minimize physical damage in army applications, reduce drag and weight penalties in airborne applications and reduce the visual and RF signatures of the communication node. This dissertation is concerned with a new class of antennas called Magneto-Dielectric wire antennas (MDWA) that provide an ideal solution to this ever-present and growing need. Magneto-dielectric structures (μr>1;εr>1) can partially guide electromagnetic waves and radiate them by leaking off the structure or by scattering from any discontinuities, much like a metal antenna of the same shape. They are attractive alternatives to conventional whip and blade antennas because they can be placed conformal to a metallic ground plane without any performance penalty. A two pronged approach is taken to analyze MDWAs. In the first, antenna circuit models are derived for the prototypical dipole and loop elements that include the effects of realistic dispersive magneto-dielectric materials of construction. A material selection law results, showing that: (a) The maximum attainable efficiency is determined by a single magnetic material parameter that we term the hesitivity: Closely related to Snoek's product, it measures the maximum magnetic conductivity of the material. (b) The maximum bandwidth is obtained by placing the highest amount of μ" loss in the frequency range of operation. As a result, high radiation efficiency antennas can be obtained not only from the conventional low loss (low μ") materials but also with highly lossy materials (tan(δm)>>1). The second approach used to analyze MDWAs is through solving the Green function problem of the infinite magneto-dielectric cylinder fed by a current loop. This solution sheds light on the leaky and guided waves supported by the magneto-dielectric structure and leads to useful design rules connecting the permeability of the material to the cross sectional area of the antenna in relation to the desired frequency of operation. The Green function problem of the permeable prolate spheroidal antenna is also solved as a good approximation to a finite cylinder.

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Date Created
2013

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Output bandwidth limitations of basestation power amplifier design and its implementation using Doherty amplifier

Description

This thesis is a study of Bandwidth limitation of basestation power amplifier and its Doherty application. Fundamentally, bandwidth of a power amplifier (PA) is limited by both its input and output prematch networks and its Doherty architecture, specifically the impedance

This thesis is a study of Bandwidth limitation of basestation power amplifier and its Doherty application. Fundamentally, bandwidth of a power amplifier (PA) is limited by both its input and output prematch networks and its Doherty architecture, specifically the impedance inverter between the main and auxiliary amplifier. In this study, only the output prematch network and the Doherty architecture follows are being investigated. A new proposed impedance inverter in the Doherty architecture exhibits an extended bandwidth compared to traditional quarterwave line.

Base on the loadline analysis, output impedance of the power amplifier can be represented by a loadline resistor and an output shunt capacitor. Base on this simple model, the maximum allowed bandwidth of the output impedance of the power amplifier can be estimated using the Bode-Fano method. However, since power amplifier is in fact nonlinear, harmonic balance simulation is used to loadpull the device across a broad range of frequencies. Base on the simulated large signal impedance at maximum power, the prematch circuitry can be designed. On a system level, the prematch power amplifier is used in Doherty amplifier. Two different prematch circuitries, T- section and shunt L methods are investigated along with their comparison in the Doherty architecture at both back off power and peak power condition. The last section of the thesis will be incorporating the proposed impedance inverter structure between the main and auxiliary amplifiers.

The simulated results showed the shunt L prematch topology has the least impedance dispersion across frequency. Along with the new impedance inverter structure, the 65% efficiency bandwidth improves by 50% compared to the original impedance inverter structure at back off power level.

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Date Created
2014

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Fast numerical algorithms for 3-D scattering from PEC and dielectric random rough surfaces in microwave remote sensing

Description

We present fast and robust numerical algorithms for 3-D scattering from perfectly electrical conducting (PEC) and dielectric random rough surfaces in microwave remote sensing. The Coifman wavelets or Coiflets are employed to implement Galerkin’s procedure in the method of moments

We present fast and robust numerical algorithms for 3-D scattering from perfectly electrical conducting (PEC) and dielectric random rough surfaces in microwave remote sensing. The Coifman wavelets or Coiflets are employed to implement Galerkin’s procedure in the method of moments (MoM). Due to the high-precision one-point quadrature, the Coiflets yield fast evaluations of the most off-diagonal entries, reducing the matrix fill effort from O(N^2) to O(N). The orthogonality and Riesz basis of the Coiflets generate well conditioned impedance matrix, with rapid convergence for the conjugate gradient solver. The resulting impedance matrix is further sparsified by the matrix-formed standard fast wavelet transform (SFWT). By properly selecting multiresolution levels of the total transformation matrix, the solution precision can be enhanced while matrix sparsity and memory consumption have not been noticeably sacrificed. The unified fast scattering algorithm for dielectric random rough surfaces can asymptotically reduce to the PEC case when the loss tangent grows extremely large. Numerical results demonstrate that the reduced PEC model does not suffer from ill-posed problems. Compared with previous publications and laboratory measurements, good agreement is observed.

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Date Created
2016

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Micromachined acoustic programmable tunable finite impulse response (FIR) filters for microwave applications

Description

This dissertation proposes a miniature FIR filter that works at microwave frequencies, whose filter response can ideally be digitally programmed. Such a frequency agile device can find applications in cellular communications and wireless networking. The basic concept of the FIR

This dissertation proposes a miniature FIR filter that works at microwave frequencies, whose filter response can ideally be digitally programmed. Such a frequency agile device can find applications in cellular communications and wireless networking. The basic concept of the FIR filter utilizes a low loss acoustic waveguide of appropriate geometry that acts as a traveling wave tapped-delay line. The input RF signal is applied by an array of capacitive transducers at various locations on the acoustic waveguide at one end that excites waves of a propagating acoustic mode with varying spatial delays and amplitudes which interfere as they propagate. The output RF signal is picked up at the other end of the waveguide by another array of capacitive transducers. Tuning of the FIR filter coefficients is realized by controlling the DC voltage profile applied to the individual transducers which essentially shapes the overall filter response. Equivalent circuit modeling of the capacitive transducer, acoustic waveguide and transducer-line coupling is presented in this dissertation. A theoretical model for the filter is developed from a general theory of an array of transducers exciting a waveguide and is used to obtain a set of filter design equations. A MATLAB based circuit simulator is developed to simulate the filter responses. Design parameters and simulation results obtained for an example waveguide structure are presented and compared to the values estimated by the theoretical model. A waveguide structure utilizing the Rayleigh-like mode of a ridge is then introduced. A semi-analytical method to obtain propagating elastic modes of such a ridge waveguide etched in an anisotropic crystal is presented. Microfabrication of a filter based on ridges etched in single crystal Silicon is discussed along with details of the challenges faced. Finally, future work and a few alternative designs are presented that can have a better chance of success. Analysis and modeling work to this point has given a good understanding of the working principles, performance tradeoffs and fabrication pitfalls of the proposed device. With the appropriate acoustic waveguide structure, the proposed device could make it possible to realize miniature programmable FIR filters in the GHz range.

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Date Created
2013

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Surface enhanced fluorescence: a classic electromagnetic approach

Description

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

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.

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Date Created
2013

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Thermal flow sensors for intravascular shear stress analysis

Description

This thesis investigated two different thermal flow sensors for intravascular shear stress analysis. They were based on heat transfer principle, which heat convection from the resistively heated element to the flowing fluid was measured as a function of the changes

This thesis investigated two different thermal flow sensors for intravascular shear stress analysis. They were based on heat transfer principle, which heat convection from the resistively heated element to the flowing fluid was measured as a function of the changes in voltage. For both sensors, the resistively heated elements were made of Ti/Pt strips with the thickness 0.12 µm and 0.02 µm. The resistance of the sensing element was measured at approximately 1.6-1.7 kohms;. A linear relation between the resistance and temperature was established over the temperature ranging from 22 degree Celsius to 80 degree Celsius and the temperature coefficient of resistance (TCR) was at approximately 0.12 %/degree Celsius. The first thermal flow sensor was one-dimensional (1-D) flexible shear stress sensor. The structure was sensing element sandwiched by a biocompatible polymer "poly-para-xylylene", also known as Parylene, which provided both insulation of electrodes and flexibility of the sensors. A constant-temperature (CT) circuit was designed as the read out circuit based on 0.6 µm CMOS (Complementary metal-oxide-semiconductor) process. The 1-D shear stress sensor suffered from a large measurement error. Because when the sensor was inserted into blood vessels, it was impossible to mount the sensor to the wall as calibrated in micro fluidic channels. According to the previous simulation work, the shear stress was varying and the sensor itself changed the shear stress distribution. We proposed a three-dimensional (3-D) thermal flow sensor, with three-axis of sensing elements integrated in one sensor. It was in the similar shape as a hexagonal prism with diagonal of 1000 µm. On the top of the sensor, there were five bond pads for external wires over 500 µm thick silicon substrate. In each nonadjacent side surface, there was a bended parylene branch with one sensing element. Based on the unique 3-D structure, the sensor was able to obtain data along three axes. With computational fluid dynamics (CFD) model, it is possible to locate the sensor in the blood vessels and give us a better understanding of shear stress distribution in the presence of time-varying component of blood flow and realize more accurate assessment of intravascular convective heat transfer.

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Date Created
2011

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Analysis, simulation and measurements of CBS antennas loaded with non-uniformly biased ferrite material

Description

When ferrite materials are used in antenna designs, they introduce some interesting and unique performance characteristics. One of the attractive features, for example, is the ability to reconfigure the center frequency of the antenna. In addition, ferrite materials also introduce

When ferrite materials are used in antenna designs, they introduce some interesting and unique performance characteristics. One of the attractive features, for example, is the ability to reconfigure the center frequency of the antenna. In addition, ferrite materials also introduce a number of challenges in the modeling and simulation of such antennas. In order for the ferrite material to be useful in an antenna design, it usually is subjected to an external magnetic field. This field induces the internal magnetic field inside the ferrite material. The internal field plays a pivotal role in the radiation characteristics of the antenna. Thus, from the numerical point of view, accurate computation of this field is critical to the overall accuracy of the analysis. Usually the internal field is non-uniform and its computation is often a rather complex and non-trivial task. Therefore, to facilitate the modeling, simplifying assumptions, which introduce some kind of averaging, are often made. In this study, ferrite-loaded cavity-backed slot antennas are used to demonstrate that averaging procedures can lead to very unsatisfactory results. For instance, it is common practice to assume that the external field is uniform by averaging its distribution. One of the pivotal points in this study is the demonstration that the external magnetic field plays a very significant role and should be included in the modeling without averaging, if the accurate results are to be attained. Results presented in this study clearly support this argument. A procedure which avoids such averaging is presented and verified by comparing simulations with measurements. In contrast to the previous formulations, the modeling methodology developed in this dissertation leads to accurate results which compare very well with measurements for both uniform and non-uniform field distributions. The utility of this methodology is especially evident for the case when the magnetic field is severely non-uniform.

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Date Created
2012

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High impedance surface using a loop with negative impedance elements

Description

Antennas are required now to be compact and mobile. Traditional horizontally polarized antennas are placed in a quarter wave distance from a ground plane making the antenna system quite bulky. High impedance surfaces are proposed for an antenna ground in

Antennas are required now to be compact and mobile. Traditional horizontally polarized antennas are placed in a quarter wave distance from a ground plane making the antenna system quite bulky. High impedance surfaces are proposed for an antenna ground in close proximity. A new method to achieve a high impedance surface is suggested using a metamaterial comprising an infinite periodic array of conducting loops each of which is loaded with a non-Foster element. The non-Foster element cancels the loop's inductance resulting in a material with high effective permeability. Using this material as a spacer layer, it is possible to achieve a high impedance surface over a broad bandwidth. The proposed structure is different from Sievenpiper's high impedance surface because it has no need for a capacitive layer. As a result, however, it does not suppress the propagation of surface wave modes. The proposed structure is compared to another structure with frequency selective surface loaded with a non-Foster element on a simple spacer layer. In particular, the sensitivity of each structure to component tolerances is considered. The proposed structure shows a high impedance surface over broadband frequency but is much more sensitive than the frequency selective surface structure.

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Date Created
2010

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Development of deformable electronics using microelectromechanical systems (MEMS) based fabrication technologies

Description

This dissertation presents my work on development of deformable electronics using microelectromechanical systems (MEMS) based fabrication technologies. In recent years, deformable electronics are coming to revolutionize the functionality of microelectronics seamlessly with their application environment, ranging from various consumer electronics

This dissertation presents my work on development of deformable electronics using microelectromechanical systems (MEMS) based fabrication technologies. In recent years, deformable electronics are coming to revolutionize the functionality of microelectronics seamlessly with their application environment, ranging from various consumer electronics to bio-medical applications. Many researchers have studied this area, and a wide variety of devices have been fabricated. One traditional way is to directly fabricate electronic devices on flexible substrate through low-temperature processes. These devices suffered from constrained functionality due to the temperature limit. Another transfer printing approach has been developed recently. The general idea is to fabricate functional devices on hard and planar substrates using standard processes then transferred by elastomeric stamps and printed on desired flexible and stretchable substrates. The main disadvantages are that the transfer printing step may limit the yield. The third method is "flexible skins" which silicon substrates are thinned down and structured into islands and sandwiched by two layers of polymer. The main advantage of this method is post CMOS compatible. Based on this technology, we successfully fabricated a 3-D flexible thermal sensor for intravascular flow monitoring. The final product of the 3-D sensor has three independent sensing elements equally distributed around the wall of catheter (1.2 mm in diameter) with 120° spacing. This structure introduces three independent information channels, and cross-comparisons among all readings were utilized to eliminate experimental error and provide better measurement results. The novel fabrication and assembly technology can also be applied to other catheter based biomedical devices. A step forward inspired by the ancient art of folding, origami, which creating three-dimensional (3-D) structures from two-dimensional (2-D) sheets through a high degree of folding along the creases. Based on this idea, we developed a novel method to enable better deformability. One example is origami-enabled silicon solar cells. The solar panel can reach up to 644% areal compactness while maintain reasonable good performance (less than 30% output power density drop) upon 40 times cyclic folding/unfolding. This approach can be readily applied to other functional devices, ranging from sensors, displays, antenna, to energy storage devices.

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Created

Date Created
2014

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Gain and loss factor for conical horns, and impact of ground plane edge diffractions on radiation patterns of uncoated and coated circular aperture antennas

Description

Horn antennas have been used for over a hundred years. They have a wide variety of uses where they are a basic and popular microwave antenna for many practical applications, such as feed elements for communication reflector dishes on satellite

Horn antennas have been used for over a hundred years. They have a wide variety of uses where they are a basic and popular microwave antenna for many practical applications, such as feed elements for communication reflector dishes on satellite or point-to-point relay antennas. They are also widely utilized as gain standards for calibration and gain measurement of other antennas.

The gain and loss factor of conical horns are revisited in this dissertation based on

spherical and quadratic aperture phase distributions. The gain is compared with published classical data in an attempt to confirm their validity and accuracy and to determine whether they were derived based on spherical or quadratic aperture phase distributions. In this work, it is demonstrated that the gain of a conical horn antenna obtained by using a spherical phase distribution is in close agreement with published classical data. Moreover, more accurate expressions for the loss factor, to account for amplitude and phase tapers over the horn aperture, are derived. New formulas for the design of optimum gain conical horns, based on the more accurate spherical aperture phase distribution, are derived.

To better understand the impact of edge diffractions on aperture antenna performance, an extensive investigation of the edge diffractions impact is undertaken in this dissertation for commercial aperture antennas. The impact of finite uncoated and coated PEC ground plane edge diffractions on the amplitude patterns in the principal planes of circular apertures is intensively examined. Similarly, aperture edge diffractions of aperture antennas without ground planes are examined. Computational results obtained by the analytical model are compared with experimental and HFSS-simulated results for all cases studied. In addition, the impact of the ground plane size, coating thickness, and relative permittivity of the dielectric layer on the radiation amplitude in the back region has been examined.

This investigation indicates that the edge diffractions do impact the main forward lobe pattern, especially in the E plane. Their most significant contribution appears in far side and back lobes. This work demonstrates that the finite edge contributors must be considered to obtain more accurate amplitude patterns of aperture antennas.

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Date Created
2014