Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Subject: search.filters.subject.Microwave

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    WAVE CHAOTIC EXPERIMENTS AND MODELS FOR COMPLICATED WAVE SCATTERING SYSTEMS
    (2013) Yeh, Jen-Hao; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Wave scattering in a complicated environment is a common challenge in many engineering fields because the complexity makes exact solutions impractical to find, and the sensitivity to detail in the short-wavelength limit makes a numerical solution relevant only to a specific realization. On the other hand, wave chaos offers a statistical approach to understand the properties of complicated wave systems through the use of random matrix theory (RMT). A bridge between the theory and practical applications is the random coupling model (RCM) which connects the universal features predicted by RMT and the specific details of a real wave scattering system. The RCM gives a complete model for many wave properties and is beneficial for many physical and engineering fields that involve complicated wave scattering systems. One major contribution of this dissertation is that I have utilized three microwave systems to thoroughly test the RCM in complicated wave systems with varied loss, including a cryogenic system with a superconducting microwave cavity for testing the extremely-low-loss case. I have also experimentally tested an extension of the RCM that includes short-orbit corrections. Another novel result is development of a complete model based on the RCM for the fading phenomenon extensively studied in the wireless communication fields. This fading model encompasses the traditional fading models as its high-loss limit case and further predicts the fading statistics in the low-loss limit. This model provides the first physical explanation for the fitting parameters used in fading models. I have also applied the RCM to additional experimental wave properties of a complicated wave system, such as the impedance matrix, the scattering matrix, the variance ratio, and the thermopower. These predictions are significant for nuclear scattering, atomic physics, quantum transport in condensed matter systems, electromagnetics, acoustics, geophysics, etc.
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    Modeling and Design of Microwave-Millimeterwave Filters and Multiplexers
    (2006-11-27) Zhang, Yunchi; Zaki, Kawthar A.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Modern communication systems require extraordinarily stringent specifications on microwave and millimeter-wave components. In mobile and integrated communication systems, miniature, ultra-wideband and high performance filters and multiplexers are required for microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs). In satellite communications and wireless base stations, small volume, high quality, high power handling capability and low cost filters and multiplexers are required. In order to meet these requirements, three aspects are mainly pursued: design innovations, precise CAD procedures, and improved manufacturing technologies. This dissertation is, therefore, devoted to creating novel filter and multiplexer structures, developing full-wave modeling and design procedures of filters and multiplexers, and integrating waveguide structures for MICs and MMICs in Low Temperature Co-fired Ceramic (LTCC) technology. In order to realize miniature and broadband filters, novel multiple-layer coupled stripline resonator structures are proposed for filter designs. The essential of the resonators is investigated, and the design procedure of the filters is demonstrated by examples. Rigorous full-wave mode matching program is developed to model the filters and optimize the performance. The filters are manufactured in LTCC technology to achieve high-integration. In order to obtain better quality than planar structures, new ridge waveguide coupled stripline resonator filters and multiplexers are introduced for LTCC applications. Planar and waveguide structures are combined in such filter and multiplexer designs to improve the loss performance. A rigorous CAD procedure using mode matching technique is developed for the modeling and design. To design wideband multiplexers for LTCC applications, ridge waveguide divider junctions are presented to achieve wideband matching performance. Such junctions and ridge waveguide evanescent-mode filters are cascaded together to realize the multiplexer designs. The design methodology, effects of spurious modes and LTCC manufacturing procedure are discussed. Additional important issues of microwave filter and multiplexer designs addressed in this dissertation are: (1) Systematic approximation, synthesis and design procedures of multiple-band coupled resonator filters. Various filter topologies are created by analytical methods, and utilized in waveguide and dielectric resonator filter designs. (2) Dual-mode filter designs in circular and rectangular waveguides. (3) Systematic tuning procedure of quasi-elliptic filters. (4) Improvement of filter spurious performance by stepped impedance resonators (SIRs). (5) Multipaction effects in waveguide structures for space applications.
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    Measurements of Doping-Dependent Microwave Nonlinearities in High-Temperature Superconductors
    (2004-04-26) Lee, Sheng-Chiang; Anlage, Steven M.; Physics
    I first present the design and use of a near-field permeability imaging microwave microscope to measure local permeability and ferromagnetic resonant fields. This microscope is then modified as a near-field nonlinear microwave microscope to quantitatively measure the local nonlinearities in high-Tc superconductor thin films of YBa2Cu3O7-d (YBCO). The system consists of a coaxial loop probe magnetically coupling to the sample, a microwave source, some low- and high-pass filters for selecting signals at desired frequencies, two microwave amplifiers for amplification of desired signals, and a spectrum analyzer for detection of the signals. When microwave signals are locally applied to the superconducting thin film through the loop probe, nonlinear electromagnetic response appearing as higher harmonic generation is created due to the presence of nonlinear mechanisms in the sample. It is expected that the time-reversal symmetric (TRS) nonlinearities contribute only to even order harmonics, while the time-reversal symmetry breaking (TRSB) nonlinearities contribute to all harmonics. The response is sensed by the loop probe, and measured by the spectrum analyzer. No resonant technique is used in this system so that we can measure the second and third harmonic generation simultaneously. The spatial resolution of the microscope is limited by the size of the loop probe, which is about 500 mm diameter. The probe size can be reduced to ~ 15 mm diameter, to improve the spatial resolution. To quantitatively address the nonlinearities, I introduce scaling current densities JNL(T) and JNL'(T), which measure the suppression of the super-fluid density as , where J is the applied current density. I extract JNL(T) and JNL'(T) from my measurements of harmonic generation on YBCO bi-crystal grain boundaries, and a set of variously under-doped YBCO thin films. The former is a well-known nonlinear source which is expected to produce both second and third harmonics. Work on this sample demonstrates the ability of the microscope to measure local nonlinearities. The latter is proposed to present doping dependent TRS and TRSB nonlinearities, and I use my nonlinear microwave microscope to measure the doping dependence of these nonlinearities.