Cosmologists are interested in topological defects as a possible source for the primordial density perturbations which seeded structure formation through gravitational instability. In this thesis, the gravitational properties of various topological defects are studied in the context of low-energy string theory, a likely modification of Einstein gravity at the high energy scales prevalent in the early universe. We consider in turn global monopole, local monopole, global cosmic string and global texture defects, allowing for an arbitrary coupling of defects to the string theory dilaton. For global defects we find the following behaviour. If the dilaton is massless, this modification to general relativity generically destroys the global good behaviour of the monopole and cosmic string, making their spacetimes singular. For the texture non-singular spacetimes exist, but only for certain values of the matter-dilaton coupling, dependent on the gravitational strength of the defect; in addition, this non-singular behaviour exists only in a certain frame. In the case of a massive dilaton, the metric behaviour of these defects is similar to that found in Einstein gravity, though we find they generically induce a long-range dilaton cloud. For the local monopole, which we study only in the presence of a massless dilaton, a rich variety of behaviour is found. For particular parameter values the local monopole spacetime approximates that of an extremal dilaton black hole.

Thesis (M.S.)--University of Missouri-Columbia, 2008. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 10, 2009) Includes bibliographical references.

Yes / A type of telecommunication technology called an ultra-wideband (UWB) is used to provide a typical solution for short-range wireless communication due to large bandwidth and low power consumption in transmission and reception. Printed monopole antennas are considered as a preferred platform for implementing this technology because of its alluring characteristics such as light weight, low cost, ease of fabrication, integration capability with other systems, etc. Therefore, a compact-sized ultra-wideband (UWB) printed monopole antenna with improved gain and efficiency is presented in this article. Computer simulation technology microwave studio (CSTMWS) software is used to build and analyze the proposed antenna design technique. This broadband printed monopole antenna contains a jug-shaped radiator fed by a coplanar waveguide (CPW) technique. The designed UWB antenna is fabricated on a low-cost FR-4 substrate with relative permittivity of 4.3, loss tangent of 0.025, and a standard height of 1.6 mm, sized at 25 mm × 22 mm × 1.6 mm, suitable for wireless communication system. The designed UWB antenna works with maximum gain (peak gain of 4.1 dB) across the whole UWB spectrum of 3–11 GHz. The results are simulated, measured, and debated in detail. Different parametric studies based on numerical simulations are involved to arrive at the optimal design through monitoring the effects of adding cuts on the performance of the proposed antennas. Therefore, these parametric studies are optimized to achieve maximum antenna bandwidth with relatively best gain. The proposed patch antenna shape is like a jug with a handle that offers greater bandwidth, good gain, higher efficiency, and compact size.

No / This paper proposes a tuneable band-rejected miniaturised monopole antenna. The band-notching was achieved by printing an inner chorded crescent shape over the surface of the substrate. By placing a small varactor between the inner and outer arches, the centre frequency of each notch can be individually shifted downwards. The design of the proposed structure has a controllable rejection in the range from 2.38 to 3.87 GHz maintaining a wideband performance from 1.5 to 5 GHz based on VSWR ≤ 2. The antenna prototype was fabricated and tested. Simulated and measured results are performed and analysed. With a compact size, the proposed monopole antenna may well work as an internal antenna in a portable device.

This dissertation presents four dual-band printed monopole antennas. There are three printed monopole antennas operating in 2.4 GHz and 5.2/5.8 GHz WLAN bands, and a dual wideband printed monopole operating applies in 2.4/5.2/5.8 GHz WLAN bands and 2.5/3.5/5.5 GHz WiMAX bands. The proposed antennas mainly use two approaches to achieve dual-band and dual wideband operations. The first method uses two different resonant paths in radiating monopole to excite two desired operating frequencies. The proposed configuration using the first method is a printed G-shaped monopole antenna. The second method places the conductor-backed plane on the other side of the radiated monopole to perturb its resonant behaviors for enhancing impedance bandwidth. Three configurations using the second method are proposed. The one with the best wideband performance is a printed rectangular monopole antenna with a trapezoid conductor-backed plane. Measured results for the proposed antennas show satisfactory performances and good agreement with the simulated results.

Novel compact antenna designs for WiMAX (Worldwide interoperability for microwave access) operation in the mobile phone are presented in this thesis. In the first design, the antenna is integrated at one corner of system circuit board of the mobile phone and consists of two radiating portions; the monopole slot is formed by integrating the two portions. In the second design, two different antenna structures are used to achieve all WiMAX operating bands. Finally, in the third design, the chip antenna comprises a resonant monopole patch, a resonant shorter slot and a matching longer slot. The latter two slots are embedded within the monopole patch to achieve a compact integration. Detailed antenna designs and experimental results are presented and discussed.

The studies in this dissertation mainly utilize planar antennas for ultra-wideband antenna designs not only on the investigation of antenna performance but also towards exploiting attractive features of ultra-wideband antennas for practical applications, such as WMAN access-point antennas, omnidirectional WiMAX access-point antennas, band-notched UWB (Ultra-wideband, 3.1 ~ 10.6 GHz) antennas, and so on. To begin with, the effects of the ground-plane size and the asymmetrical ground plane on ultra-wideband antennas are studied in Chapter 2. Following up, from the conclusive results, an antenna for WMAN operation in access-point applications and an omnidirectional monopole for USB wireless network card device are proposed and analyzed. Characteristics of ultra-wideband antenna radiation in relation to the antenna's width for obtaining omnidirectional radiation are addressed. In Chapter 3, several ultra-wideband access-point antennas are presented for achieving good omnidirectional radiation in the azimuthal plane across the bandwidth. Furthermore, in Chapter 4, band-notching techniques are applied to ultra-wideband antennas for avoiding the interference between the UWB and the WLAN systems.

No description available.

In this thesis, a comparison of the body SAR value of five planar WLAN antennas including two coupled-fed shorted monopole antennas, a coupled-fed loop antenna, a monopole antenna, and a monopole slot antenna for tablet PC applications are presented. A required minimum distance between the antenna and the testing flat phantom to meet the 1-g body SAR requirement of 1.6 W/kg has been determined. Results show that the body SAR results of the coupled-fed loop antenna are lowest among the five tested antennas. Detailed results and discussion are presented in this thesis. Also, a comparison of the body SAR value of a planar antenna and an antenna with 3-D bent structure are presented.

Approved for public release; distribution is unlimited / This thesis investigates the viability of a new method for numerically computing the input impedance and the currents on simple antenna structures. This technique considers the antenna between two ground planes and uses multiregion cylindrical harmonic expansions with tangential field continuity to obtain the surface currents and input impedance. The computed results are compared to the results obtained from the Numerical Electromagnetics Code for various physical parameters to assess computational accuracy. / http://archive.org/details/computationofmon00hurl / Lieutenant, United States Navy