Browsing Master's Theses by Subjects
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Novel Antennas, Matching Circuits, and Fabrication Techniques at HF and Microwave FrequenciesThis thesis focuses on the investigation of several novel antennas including electrically small antennas for HF communication system, as well as the applications of gradient index lens based broadband multiple-beam system, and electromagnetic cloak structures in printed technology. Modern-day wireless communication systems have developed interest in the field of low-profile broadband antennas. The design of electrically small antennas (ESA) presents numerous challenges, primarily due to inherently low impedance and narrow bandwidths. Improving these performance characteristics becomes even more challenging in the high frequency (HF) band due to longer wavelengths and corresponding antenna physical dimensions. In this thesis, we propose electrically-small vertically-polarized normal mode helical antenna (NMHA), about λ/50 at the lowest frequency of operation, to facilitate robust long-range HF-band communications (3 – 30 MHz). To overcome the potential issues related to electrically-small NMHA such as impedance matching, bandwidth and radiation efficiency, passive and active impedance matching techniques are investigated. Three types of matching networks are proposed, designed and experimentally demonstrated. These include passive narrowband electronically-switched LC matching network, broadband transformers and active non-Foster broadband matching circuit. In addition, performance of electrically-small helical antenna (ESHA) matched system in terms of received signal power strength, signal-to-noise ratio (SNR), and signal intelligibility is evaluated with the help of outdoor field test measurements. Many potential areas of application such as satellite communication, air-traffic control, air-based tracking and surveillance, marine navigation, and automotive radar require highly-directional wide-angle beam scanning with minimum pattern deformation and broadband behavior, in addition to lower-cost and weight considerations. In view of this, another area of concentration addressed by the thesis is towards the development of multiple-beam Luneburg lens antenna system. The additive manufactured 3D graded-index Luneburg lens is employed for this application. Using the special property of a Luneburg lens that every point on the surface of the lens is the focal point of a plane wave incident from the opposite side, compact conformal dual-polarized all-metal Vivaldi feed array, with its phase center close to the lens surface, is proposed to realize the full potential of Luneburg lens for practical wireless applications ranging from 3 – 6 GHz. Lastly, this thesis discusses another interesting topic about electromagnetic invisibility and cloaking technology being applied to printed antennas in order to reduce mutual near-field coupling, based on the concept of mantle cloaking method. Two microstrip-fed monopole antennas placed in the near-field of each other, resonating at slightly different frequencies, become invisible to each other by cloaking the radiation part of each antenna. The cloak structure is realized by a conformal elliptical metasurface formed by confocal printed arrays of sub-wavelength periodic elements, partially embedded in the substrate. The existence of the metasurfaces leads to restoration of the radiation patterns of the antennas as if they were isolated. Finally, the fabrication of near-field cloaked prototype is carried out using advanced 3D printing techniques.