Maldonado-Vargas, Jayson

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    Design and characterization of cavity-backed asymmetric annular ring slot antenna and UWB cavity-backed two-arm modified compound power-Archimedean slot spiral antenna for body centric wireless communications applications
    (2015) Maldonado-Vargas, Jayson; Rodríguez-Solís, Rafael A.; College of Engineering; Juan García, Eduardo; Filipovic, Dejan S.; Colom Ustariz, José; Department of Electrical and Computer Engineering; Ocasio Gonzalez, Victor A.
    In this thesis, two types of slot antennas are studied, designed and characterized for the purpose of achieving Ultra-wideband (UWB) capabilities when in close proximity to the human body. The antennas are first designed considering free space conditions and then studied with common human body models from the literature. First an asymmetric annular ring slot antenna fed with coplanar waveguide (CPW) and with a cylindrical cavity and bottom ground plane operating at 7 GHz is designed and characterized using Design of Experiments (DOE) techniques to study the influence of the antennas dimensions with respect to bandwidth, center frequency bandwidth, and broadside gain. The proposed characterization consists of full factorial design with six center points and 10 axial runs for estimating higher order coefficients performed using ANSYS HFSS. A total of three different DOE cases are done to obtained a detailed statistical characterization with linear regression models. Different approaches to increase bandwidth on the characterized antenna are done afterwards for reaching 50 ohms impedance match from 3.1-10.6 GHz while sacrificing broadside gain performance throughout the band. A detailed design analysis and simple characterization of a compound slot spiral antenna composed of an Archimedean or Logarithmic geometry at the center and a power spiral geometry tapered at the outside is performed as the second type of slot antenna and as the preferred choice for achieving wideband characteristics. The compound slot spiral antenna was design on a Rogers RT/Duroid 5880 substrate with dielectric constant and dielectric thickness of 2.2 and 0.79 mm respectively. The antenna contains a shallow cavity located at the slot side for retrieving unidirectional radiation pattern with the purpose of minimizing interactions between antenna and human body. Resistive terminations are inserted at the open end of each spiral arm for reduction of the dispersive characteristics presented in spirals antennas as well as improve axial ratio and reflection coefficient at low frequencies when a cavity is inserted. Three different types of balun feeds are discussed and analyzed ending with the choice of a coaxial bundle feed configuration and a broadband 180° hybrid coupler. This balun avoid the use of long vertical balun and allow for easier implementation of shallow cavity. Two possible ways of implementing this type of balun in the antenna are investigated. Results from simulation and measurements from a prototype show that the antenna behavior remains stable throughout the FCC UWB band (3.1-10.6 GHz) when in close proximity to the human body. Thus, this type of antenna is a viable candidate for body centric wireless communications.