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<p> The allocation of 7.5 GHz of bandwidth by the Federal Communication Commission (FCC) for ultra-wideband (UWB) applications has provided an exciting and a challenging opportunity to design short range wireless communication and microwave imaging systems. To fully realize the potential of the UWB, communication and microwave imaging systems are required to operate over the entire UWB frequency band. The combination of the wide bandwidth requirement and the target application of the UWB systems have led to a surge of interest in designing of novel integrated circuits and antennas for the UWB applications. In any wireless communication and microwave imaging system, the antenna has a fundamental effect on the overall performance of the system, and as a result, it has attracted considerable research interest.</p> <p> This thesis focuses on the design of UWB antennas that are suitable for UWB cancer detection and wireless communication systems. Two planar antennas, one a printed monopole antenna, and the other, a printed slot antenna fed with a coplanar waveguide, are presented in this work. First, the antennas have been designed to operate in air, making them suitable for the UWB wireless short range communication applications. Measurement and simulation results indicate that both antennas achieve input impedance matching in a bandwidth of more than 7.5 GHz. The effect on the input matching of the antennas due to the variation in their geometrical parameters has been studied and both antennas have been fully characterized in air. Next, the two antennas have been redesigned to operate in a coupling medium for medical imaging applications. Both antennas achieve return loss of less than -10 dB over the entire UWB spectrum. Also, the antennas have been fully characterized while operating in the coupling medium and in proximity to a human body model. Finally, a two-element antenna array, based on the printed monopole antenna and the printed slot antenna, in co- and cross-polarized array formation, has been designed. The tumor detection capabilities of all antenna arrays for different scenarios have been studied through electromagnetic simulation and measurements.</p> / Thesis / Master of Applied Science (MASc)

No / Two novel reduced-size, printed spiral antennas are proposed for use in personal communications mobile terminals exploiting the “big low earth orbit” (Big-LEO) satellite system (uplink 1.61–1.63 GHz; downlink 2.48–2.5 GHz). The two proposed antenna give 3.12―6.25% bandwidth at lower resonant mode of 1600MHz, while at the higher resonant mode of 2450MHz a bandwidth of around 6% is obtained. The experimental and simulated return losses of the proposed antennas show good agreement. The computed and measured gains, and axial ratios are presented, showing that the performance of the proposed two antennas meets typical specifications for the intended applications.

An indirect impedance measurement approach that does not require direct cable attachment or large space using a two-port network is presented. Using a straight wire monopole as an interrogating antenna and measured impedances of three calibration standards, the input impedance of a small spherical helix dipole over a ground plane is retrieved. It is found that accurate result is obtained around the dipole resonance frequency. The accuracy and sources of error are discussed.

Smart antenna technology is a promising means to overcome signal impairments in wireless personal communications. When spatial signal processing achieved through smart antennas is combined with temporal signal processing, the space-time processing can mitigate interference and multipath to yield higher network capacity, coverage, and quality. In this dissertation, we propose a dual smart antenna system incorporated into handsets for the third generation wireless personal communication systems in which the two antennas are separated by a quarter wavelength (3.5 cm). We examine the effectiveness of a dual smart antenna system with diversity and adaptive combining schemes and propose a new combining scheme called hybrid combining. The proposed hybrid combiner combines diversity combiner and adaptive combiner outputs using maximal ratio combining (MRC). Since these diversity combining and adaptive combining schemes exhibit somewhat opposite and complementary characteristics, the proposed hybrid combining scheme aims to exploit the advantages of the two schemes. To model dual antenna signals, we consider three channel models: loosely correlated fading channel model (LCFCM), spatially correlated fading channel model (SCFCM), and envelope correlated fading channel model (ECFCM). Each antenna signal is assumed to have independent Rayleigh fading in the LCFCM. In the SCFCM, each antenna signal is subject to the same Rayleigh fading, but is different in the phase due to a non-zero angle of arrival (AOA). The LCFCM and the SCFCM are useful to evaluate the upper and the lower bounds of the system performance. To model the actual channel of dual antenna signals lying in between these two channel models, the ECFCM is considered. In this model, two Rayleigh fading antenna signals for each multipath are assumed to have an envelope correlation and a phase difference due to a non-zero AOA. To obtain the channel profile, we adopted not only the geometrically based single bounce (GBSB) circular and elliptical models, but also the International Telecommunication Union (ITU) channel model. In this dissertation, we also propose a new generalized selection combining (GSC) method called minimum selection GSC (MS-GSC) and an adaptive rake combining scheme to reduce the power consumption of mobile rake receivers. The proposed MS-GSC selects a minimum number of branches as long as the combined SNR is maintained larger than a given threshold. The proposed adaptive rake combining scheme which dynamically determines the threshold values is applicable to the three GSC methods: the absolute threshold GSC, the normalized threshold GSC, and the proposed MS-GSC. Through simulation, we estimated the effectiveness of the proposed scheme for a mobile rake receiver for a wideband CDMA system. We also suggest a new power control strategy to maximize the benefit of the proposed adaptive scheme. / Ph. D.

This report focuses on the application of adaptive arrays to the Advanced Mobile Phone Service (AMPS) and Code Division Multiple Access (CDMA) cellular systems. Adaptive arrays have been proposed as early as in the 1960s to improve the signal quality, but most of its applications were restricted to defense purposes. Recently, there has been a surge in interest of applying adaptive arrays for cellular systems. This work introduces new blind adaptive array algorithms for AMPS and CDMA signals. The theoretical capacity limit using an adaptive array at the base station for an AMPS cellular system is derived in this work. One of the significant contributions in this research is a macrocell channel model which provides angle-of-arrival (AOA) statistics of the multipath components. Practical issues involved in the implementation of an adaptive array are addressed and the author's implementation of an 8-element adaptive array operating at 2.05 GHz is explained. This research also analyzes the capacity that can be o ered by an adaptive array in a system where CDMA users co-exist with existing AMPS users. A novel cellular CDMA system which exploits adaptive arrays is introduced and the capacity o ered by this system is compared with existing and other systems exploiting spatial dimension. / Ph. D.

Modifications of large reflector antennas, such that their observing capabilities are enhanced in the range of about 10-500~MHz without affecting operation of the pre-existing higher-frequency systems, are addressed in this dissertation. The major contributions of this dissertation can be divided into two parts: 1) designing new low frequency feeds, and 2) developing new analysis methodologies which, as opposed to traditional techniques, are suitable for analyzing low frequency systems. First, we consider the performance of existing schemes that provide low frequency capability. Then, a new class of dipole-based low frequency feeds - namely, the ``distributed feed array'' - is designed to cover the frequency range of interest without affecting operation at higher frequencies. As an example, distributed feed arrays are designed for the Expanded Very Large Array (EVLA) to cover the range of 50-250~MHz. A method of moments (MoM) model of an EVLA antenna is developed for this purpose. The new design shows performance comparable to the existing 4 m system on the EVLA in the range of 50-88~MHz, and introduces observing capabilities in the range of 110-250~MHz (currently not covered by the EVLA). Moreover, the blockage presented to the existing EVLA L-band system is reduced significantly when the existing 4 m system is replaced by the proposed system. At low frequencies, external noise can be a significant or dominant contribution to the total noise of the system. This, combined with mutual coupling between the array elements of the distributed feed array, makes it difficult to predict the sensitivity of these systems. This dissertation describes a system model and procedure for estimating the system equivalent flux density (SEFD) - a useful and meaningful metric of the sensitivity of a radio telescope - that accounts for these issues. We consider the efficiency of methods other than MoM - in particular, Physical Optics (PO), Uniform Geometrical Theory of Diffraction (UTD), and hybrid methods - for accelerated computation at low frequencies. A method for estimating the blockage presented by low frequency systems to the pre-existing higher-frequency systems is also described. / Ph. D.

Strain-mediated magnetoelastic composite materials are being considered for communication in lossy environments. Their consideration is attributable to predictions stating order of magnitude improvements over current antenna technology. The magnetic antenna design considered herein consists of three layers: 1) a piezoelectric layer, 2) a linear elastic layer, and 3) a magnetoelastic layer. The antenna operates by mediating strain through the device in a resonant bending mode. The magnetoelastic layer is stressed which results in a changing magnetization ultimately leading to a changing magnetostatic field in free space which acts as a signal for information transfer. In order to prove the efficacy of this approach finite element models have been developed to aid in the design and optimization process. Where these models fall short is in their overall run-time to fully resolve the coupled dynamics. It is for this reason that the work presented in this thesis focuses on the development of a figure of merit capable of predicting optimal bias conditions and geometries needing only the data from a static bias study from FEA. The material level magnetomechanical coupling factor is chosen as the foundation for the figure of merit. The figure of merit is then augmented to include structure level information regarding the demagnetizing field and the non-uniform stress distribution. The main results presented are the effects of including demagnetization and stress distributions, and most importantly the ability of the metric to predict the change in magnetization of the device. It is shown that for aspect ratios greater than roughly 2.5 the metric trends the same as the change in magnetization predicted by finite element simulations. The region of disagreement between the metric and the fully resolved finite element simulation is explained by tying back to underlying assumptions made during the formulation of the magnetometric demagnetization factor used in the analysis. The case is made for the figure of merit to be included in the analysis of strain-mediated antennas for its ability to find optimum designs while reducing the overall simulation run-time by an order of magnitude. / Master of Science / Many communication devices are readily available however there are a few key gaps in communication technology that are yet to be filled. Notably, communication in lossy environments using small scale, low frequency, devices has proven difficult due to the fundamental limits of antennas (a cell phone cannot communicate into a mine shaft for search and rescue operations, nor can they communicate underwater to submarines or divers for instance). A promising new approach of communication using smart magnetic materials is under consideration in this thesis. Specifically, the goal herein is to develop an analysis tool capable of predicting device performance without having to run computationally expensive/time consuming finite element simulations. In this thesis it is shown that the analysis tool is capable of predicting device performance while reducing the necessary simulation run-time by an order of magnitude. Using this tool, researches will be able to design better prototypes; moving one step closer to portable communication in lossy environments.

The purpose of this thesis is to investigate a novel method to develop broadband microstrip (patch) antennas using substrates containing photonic crystals. Photonic crystals are a class of periodic dielectric, metallic, or composite structures that when introduced to an electromagnetic signal can exhibit a forbidden band of frequencies (or bandgap) in which the incident signal destructively interferes and thus is unable to propagate. It is proposed that such photonic crystals will reduce surface waves and prohibit the formation of substrate modes, which are commonly known inhibitors of patch antenna designs. By reducing or eliminating the effects of these electromagnetic inhibitors with photonic crystals, a broadband response can be obtained from inherently narrowband antennas. In addition, it is also proposed that the behavior of the photonic crystals will lead to a reduction in pattern sidelobes resulting in improvements in radiation pattern front-to-back ratio and overall antenna efficiency. This research is verified through analytical simulations and experimental investigations in the Virginia Tech anaechoic chamber. / Master of Science

A goal of indoor wireless communication is to strategically place RF base stations to obtain optimum signal coverage at the lowest cost and power. Traditionally, transceiver locations have been selected by human experts who rely on experience and heuristics to obtain a near-optimum placement. Current methods depend on involved on-site communication measurements and crude statistical modeling of the obtained data which is time consuming and prohibitive in cost. Given the inherent variability of the indoor environment, such a method often yields poor efficiency. As an example, it is possible that more power than required or extra number of transceivers were used. This thesis describes an interactive software system that can be used to aid transceiver placement. The tool is easy to use and is targeted at users who are not experts in wireless communication system design. Once the transceiver locations are selected by the user within a graphical floor plan, the system uses simple path-loss models to predict coverage regions for each transceiver. The coverage regions are highlighted to indicate expected coverage. Earlier work assumed isotropic transceivers and had limited directional transmitter support. This thesis describes how the tool has been enhanced to support a wide range of 3D antenna patterns as encountered in practical situations. The tool has also been expanded to accommodate more partition types and to report area of coverage. The resulting system is expected to be very useful in the practical deployment of indoor wireless systems. / Master of Science