[pt] A presente tese estudará as modificações introduzidas nos diagramas de radiação de antenas cassegrain, em decorrência da colocação de obstáculos, como cercas, na região de campos próximos. A análise destas modificações será baseada na Teoria Geométrica da Difração. Definida a geometria do problema, serão desenvolvidos algoritmos para determinação dos pontos de reflexão e difração e, a partir destes pontos, calculados os campos espalhados. A eficiência de diversas configurações de obstáculos, na redução dos campos irradiados em certas direções, será analisada. / [en] This work will study the modifications introduced on cassegrain antenna radiation patterns, by putting obstacles, such as fences, in the near-field zone. The analisys of these modifications will be supported on the uniform Geometrical Theory of Diffraction. Defined the geometry of the problem, a mathematical model will be developed for the determination of reflection and diffraction points and, following these, the scattered fields will be computed. The performance of the antenna in the presence of obstacle will be analised.

Communication systems have remained almost unchanged since the invention of the superheterodyne receiver in 1918 by the US engineer Edwin Armstrong. With the introduction of multiple-input-multiple-output (MIMO) technologies, Index Modulation appears to be the promising technology to revolutionize the traditional radio-frequency (RF) chain. Index modulation is a high-spectrum, energy-efficient, simple digital communication technique that uses the states of the building blocks of a communication system. In this study, we have focused on the use of radiation patterns scattered by antenna arrays or a metasurface as indices that are encoded as data bits. Initially, we explore sets of 𝑁tx transmitting point source antennas located on the XY plane; we assume that every antenna has phase tunability capability. The phase, the position in space, and the size of the array determine the shape of the far-field radiation pattern. Following the antenna excitation, a set of 𝑁rx receiver antennas spread at specific locations of the spherical space measures the incoming power signal, allowing the sampling of the radiation pattern that is demodulated into information bits.This work is focused on the characterization of the measured radiation patterns under different system and channel variables and their direct effect on the Bit Error Rate.

Dans cette thèse, nous avons d'abord réalisé une analyse de la performance analytique des deux plus populaires systèmes de la radio cognitif (CR), à savoir les réseaux de radio cognitive (CRN) interweaved et underlay. Il a été montré que numériquement le comportement de chacun des approches CRN examinés est fortement dépendant des paramètres du système de base. En outre, nous avons étudié le problème de taux optimale de recevoir BF et la sélection de l'utilisateur, compte tenu de la liaison montante d'un multi-utilisateur, CRN sans priorité. Comme l'hypothèse d'une information d'état de canal (CSI) réglage, par lequel les chaînes concernées ne seraient que instantanément (resp. statistiquement) connu est, en grande partie, optimiste (resp. pessimiste), nous avons considéré un scénario de CSI mixte. Ensuite, le problème de taux des optimale de transmission BF pour un MISO underlay CRN, en supposant l'existence de CSI mixte, a ensuite été formulée. Se concentrer sur la communication de downlink, l'objectif de la conception du système était la maximisation de la capacité ergodique réalisable du système secondaire, soumis à une contrainte de taux moyen imposée sur la communication primaire. Poursuite de l'enquête du problème de précodage dernier avec la connaissance du canal distribute et mixte, nous avons développé un système de coordination, selon lequel, les émetteurs de coordonner sur la base de statistiques (covariance) des informations de la chaîne mondiale. La stratégie de pré-codage proposé a été montré à surperformer les approches classiques tirés de la littérature. Enfin, dans un cadre CRN priorité, nous avons proposé un algorithme d'affectation des pilotes. / In this thesis, we initially conducted an analytical performance analysis of two of the most popular cognitive radio (CR) schemes, namely the interweaved and the underlay cognitive radio network (CRN) approaches. It was numerically shown that the behavior of each of the examined CRN approaches is highly dependent on basic system parameters. Furthermore, we studied the problem of rate-optimal receive BF and user selection, considering the uplink of a multi-user, unprioritized CRN. As the assumption of a channel state information (CSI) setting, whereby the involved channels would be merely instantaneously (resp. statistically) known is, to a great extent, optimistic (resp. pessimistic), we considered a mixed (combined) CSI scenario. Then, the problem of rate-optimal transmit BF for a MISO underlay CRN, assuming the existence of mixed CSI, was thereafter formulated. Concentrating on downlink communication, the goal of the system’s design was the maximization of the secondary system’s achievable ergodic capacity, subject to an average rate constraint imposed on primary communication. Continuing the investigation of the latter precoding problem with mixed, distributed channel knowledge, we developed a coordination scheme, according to which, the transmitters coordinate on the basis of statistical (covariance) information of the global channel. The proposed precoding strategy was shown to outperform conventional approaches taken from the literature. Finally, within a prioritized CRN framework, we proposed a pilot assignment algorithm.

A new variation of the spherical helical antenna made of a wire wound over a hemispherical surface and backed by a conducting ground plane is introduced. A constant spacing is maintained between the turns of winding. The geometry of this antenna is fully described by the number of turns and the radius of hemispherical surface. In addition to the hemispherical geometry, truncated double spherical helices are also examined. Radiation properties of the proposed antennas are studied both theoretically and experimentally. The wire antenna code ESP (electromagnetic surface patch), which is based on the method of moments, is used to obtain simulation results. The results for far-field patterns, gain, axial ratio, bandwidth, and input impedance are presented. Several prototypes of this antenna were constructed and tested using an outdoor antenna range. Far-field patterns were measured over a wide range of frequencies. The measured and calculated radiation patterns are in good agreement. A unique property of the hemispherical helix is its broad half-power beamwidth. Furthermore, this antenna provides circular polarization and relatively high gain over a narrow frequency range. The results of this research show that, for example, a 4.5-turn hemispherical helix with a radius of 0.02 meter designed for operation around 2.8 GHz provides a half-power beamwidth of about 90 degrees, more than 9 dB gain, and less than 3 dB axial ratio over a 300 MHz bandwidth. The input impedance of the antenna is largely resistive and is about 150 ohms in the above frequency range. Compared with a full spherical helix, the hemispherical helix provides comparable radiation characteristics, but occupies only half the volume. The compact size of this antenna makes it attractive to mobile communication applications / Master of Science

Thesis: Sc. D., Massachusetts Institute of Technology, Department of Electrical Engineering, 1961. / [1970]. / Cambridge, Mass. / 3 microfiches. / Massachusetts Institute of Technology, Microreproduction Laboratory, / "September 1961." Vita. / Includes bibliographical references (leaves 136-138 ). / by Allan Carter Schell. / Sc. D.

The overwhelming prevalence of cardiac related deaths is the motivation behind this thesis to develop body centric antennas for wireless cardiac monitoring. Cardiac monitoring can diagnose a number of conditions including: arrhythmia, ischemia, premature atrial complexes, abnormal sinus rhythms, heart blocks, atrial fibrillation, and more. A body centric antenna operating within the ISM band (2.4-2.48GHz) has been designed, simulated, and tested. The simulation and testing indicate low mutual coupling between antennas of varying distances has been achieved. In addition, the simulation and testing indicate that a thin layer of skin over the test subject further reduces mutual coupling.

Dielectric properties play an influential role in the development of medical devices. Understanding the behavior of these properties and how they respond to external stimuli, such as heat, over an extended frequency has yet to be researched. The focus of this study is to examine the impact of temperature on dielectric properties from 500 MHz to 10 GHz in order to better match the antenna properties of medical applications to the dielectric properties of biological tissue in question; more specifically, microwave ablation, microwave hyperthermia, and thermal modeling of brown adipose tissue’s metabolic processes. The dielectric properties of biological tissue samples from porcine lung, liver, heart, skin, fat, and muscle as well as brown adipose tissue and white adipose tissue from rat have been tested. These results have then been used to develop medical applications involving microwave antennas.

No description available.

No description available.

No description available.