Les mesures utilisées actuellement en chambres anéchoïque, pour la caractérisation de la surface équivalente radar (SER) sont généralement perturbées, en particulier en basse fréquence lorsque les parois de la chambre génèrent des trajets multiples. En basse fréquence l’épaisseur des matériaux absorbants devient faible devant la longueur d’onde et ceux-ci ne peuvent plus être considère comme parfaitement absorbants. Dans ce cas, le champ total incident sur la cible sous test est ainsi la somme du champ incident direct et des champs réfléchis par les parois. Basé sur le principe de retournement temporel qui permet de focaliser spatialement et temporellement une onde électromagnétique, l’objectif de la thèse est de présenter une nouvelle méthode d’imagerie microonde qui profite des degrés de liberté spatio-temporels de la propagation au sein d’un milieu diffusant afin de détecter une cible avec une haute résolution. / The currently used methods for radar cross section (RCS) measurements at low frequencies encounter several disturbing phenomena, such as the multipath propagation caused by reflections from the walls. Descending in frequency, the thickness of absorbent materials becomes small compared with the wavelength and they are no longer perfectly absorbent. Therefore, the total incident field on the target becomes the sum of the incident wave and the reflected waves from the walls of the chamber. Inspired by time reversal technique which allows spatial and temporal focusing of an electromagnetic wave, the objective of the thesis is to present a novel imaging method in the microwave range which benefits from the spatio-temporal degrees of freedom provided by the transmission through a random medium in order to detect a target with high resolution.

Ces travaux de thèse ont porté sur l’étude de nouveaux matériaux pour l’absorption électromagnétique en chambre anéchoïque. Ce sujet a été imaginé à partir de l’étude d’une nouvelle matrice jusqu’alors jamais utilisée pour les absorbants électromagnétiques : la mousse époxy. Cette dernière présente en effet de nombreux avantages par rapport aux matrices habituellement utilisées dans le commerce : possibilité de réaliser des formes complexes, piégeage de la charge au cœur du matériau… Nous avons associé cette matrice avec différentes charges carbonées (noir de carbone, graphite et des fibres longues de carbone). L’association de la mousse époxy avec des fibres de carbone millimétriques a montré d’excellentes performances d’absorption pour un taux de charge très faibles : 0,5 %wt (S11 ≈ -40 dB entre 4 et 18 GHz en incidences normale et oblique). En variant la longueur des fibres de carbone, nous avons mis en évidence qu’il était possible d’améliorer les performances d’absorption en basses fréquences en utilisant des fibres plus longues. Enfin, nous avons dirigé nos recherches sur la réalisation d’un matériau absorbant ‘Vert’ à partir d’une matrice de liège. Ces nouveaux matériaux, réalisés à partir de matières biosourcées, ont montré de meilleures performances d’absorption qu’un absorbant du commerce de mêmes dimensions en incidence normale (S11 = -53 dB et S11 = - 27 dB respectivement à 4,2 GHz) et en incidence oblique (S11 = -50 dB et S11 = -30 dB respectivement à 4,2 GHz) et constituent donc des candidats potentiels pour le remplacement des matériaux absorbants du commerce actuels. / This thesis work focused on the study of new materials for electromagnetic absorption in anechoic chambers. This subject arose from the study of a new matrix which was never used for electromagnetic absorbers until then: the epoxy foam. This foam has many advantages compared to the matrices usually used in the trade like the possibility of cutting complex shapes out of them or trapping the charge in the core of the absorber...This matrix was associated to different carbonaceous load (carbon black, graphite and carbon fibers). The combination of epoxy foam with millimeter carbon fibers has shown better absorption performance at very low loading rates: 0.5 %wt (S11 ≈ -40 dB between 4 and 18 GHz under normal and oblique incidences). The use of different fiber lengths showed that it is possible to improve absorption performance at low frequencies using long carbon fibers. Finally, we directed our work on the creation of an absorbent material from a cork matrix. These new materials, made from bio-based materials, have shown better absorption performance than a commercial absorber, with the same dimensions in normal incidence (S11 = -54 dB and S11 = - 27 dB respectively at 4.26 GHz) and oblique incidence (S11 = -51 dB and S11 = -30 dB respectively at 4.26 GHz) and are therefore potential candidates for the replacement of existing commercial absorbent materials.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The purpose of this work is to study and characterize aerodynamically a free jet operating at subsonic regime and identify its acoustic signature. This study aims to analyze fundamentally the turbulent structures and the total noise produced at different Mach numbers. This kind of research is crucial to the understanding of these mechanisms of noise generation and propagation, and it is extremely important for aeronautical applications, for instance, the jet engine exhaust. The research is done by analysing the data obtained in experiments using pitot tube, hot-wire anemometer and acoustic measurements. This work also describes the experimental procedures for each step of analysis, as well as the characteristics of jet noise facilities. The data from measurements with a pitot tube was used to study the mean velocity profiles. The average properties are also analyzed with an anemometry system, likewise used to study the turbulent intensity at eleven axial lines, ranging from the centerline to the edge of the nozzle (lipline). These results are compared with literature and has verified the accuracy of hot-wire anemometers for turbulent intensities lower than 15%. The aerodynamic data are obtained for Mach numbers 0.25, 0.50 and 0.75, from the nozzle exit to thirteen diameters in the direction of the jet. The acoustic study is carried out by analyzing the sound pressure level obtained at six positions in the far field, with observer angles ranging from 40 to 110º. In this campaign more velocities are studied with Mach numbers from 0.18 to 1.00 with step of 0.05 are described. A database with the sound pressure level as a function of frequency is constructed from this information. / O objetivo deste trabalho é estudar e caracterizar aerodinamicamente um jato livre operando em regime subsônico e identificar a assinatura acústica do mesmo. Esse estudo busca analisar fundamentalmente as estruturas turbulentas e o ruído total produzido em diferentes números de Mach. Tal estudo é crucial para o entendimento desses mecanismos de geração e propagação, e encontra extrema importância para aplicações aeronáuticas, como, por exemplo, a exaustão de motores a reação (jato). A investigação é feita através da análise dos dados obtidos em experimentos utilizando tubo de pitot, anemômetro de fio-quente e ensaios acústicos. Neste trabalho também são descritos os procedimentos experimentais de cada etapa de análise, bem como as características dos laboratórios utilizados para o estudo do ruído de jato. Com os dados provenientes das medições com tubo de pitot são estudados os perfis de velocidade média. As propriedades médias também são analisadas com o sistema de anemometria, que ainda é utilizado para estudo da intensidade turbulenta em onze linhas axiais, variando da linha de centro até a borda do bocal (lipline). Estes resultados são comparados com a literatura e é constatada a acurácia dos anemômetros de fio-quente para intensidades turbulentas menores que 15%. Os dados aerodinâmicos mencionados são obtidos para números de Mach 0,25, 0,50 e 0,75, a partir da saída do bocal até treze diâmetros na direção do jato. O estudo acústico é feito através da análise do nível de pressão sonora obtido em seis posições no campo distante, com ângulos de observação variando de 40 a 110º. Diferentes velocidades também foram analisadas, desta vez, com números de Mach de 0.18 a 1.00 com passo de 0.05. Um banco de dados com o nível de pressão sonora em função da frequência é construído a partir destas informações. / Mestre em Engenharia Mecânica

Ultra wideband (UWB) technology plays a significant role in wireless communication. The complex antenna transfer function (CATF) of an UWB antenna provides important information required for better channel designs and communication systems. In this dissertation the CATF of a Double ridge guide horn (DRGH) antenna is determined and used as a standard antenna for UWB measurements. Two methods were used: the two antenna method in an anechoic chamber and a modified gain-transfer method in a compact antenna test range (CATR). Measurements were performed with a vector network analyser (VNA) in the frequency domain, in the anechoic chamber and the CATR. The distance measurements required to calculate the CATF from the S-parameter measurements were performed in the time domain. The CATF of the standard antenna was determined using two identical antennas and then it was shown that a modified gain-transfer method can be used to determine the CATF of any unknown antenna in a CATR, using the standard antenna as a reference. Some of the challenges were to obtain the correct equations and measurement method to obtain the CATF in a CATR. The standard antenna was used to investigate uncertainty contributions for the measurements in the CATR. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Electrical, Electronic and Computer Engineering / unrestricted

Electromagnetic compatibility (EMC) is an essential part in today’s society and there are more products around us that emit electromagnetic waves than ever before. To make sure that all these products function properly under all circumstances EMC testing is needed. One test that is conducted is radiated immunity testing. A susceptibility antenna is needed to perform immunity testing. This thesis aims to show that a tuneable antenna could be used for immunity testing in the frequency band 26-150 MHz and in the future replace the current antenna, which is not tuneable, used at SAAB Support and Services EMC in Östersund. A simulation program called EZNEC+ was used to simulate different antennas that were tested in the semi-anechoic chamber at SAAB. Two antenna types showed better efficiency and reached lower in frequency than SAAB’s current antenna. These antennas were a bowtie antenna and an x-shaped antenna, both extending in only two spacial directions instead of the normal three. Their drawback was a less uniform E-field at the lowest frequencies, where the E-field was much stronger to the sides compared to straight in front of the antenna. This results in a narrow lobe width, but if this drawback could be accepted or mitigated both antennas are possible replacements for the current antenna at SAAB and should be further investigated.

Cette thèse élargie le champ des applications de l’analogie microonde en adaptant les techniques de fabrication permettant de contrôler les analogues et en développant un outil versatile et précis pour réaliser les mesures. L’analogie microonde consiste à transposer à l’échelle microonde les objets dont nous souhaitons étudier la diffraction en créant un analogue respectant le même rapport dimension sur longueur d’onde, et les mêmes propriétés morphologiques électromagnétiques. Une attention particulière a été portée aux objets faiblement diffractant ayant des niveaux SER aussi faibles que -60 dBm2. A la suite de la caractérisation des réflexions parasites et de celle du bruit aléatoire perturbant les mesures, une nouvelle technique d’optimisation du paramétrage des appareils de mesure a été proposée. Elle comporte notamment un réglage des puissances de source en fonction des angles de bistatisme et un filtrage temporel, par switch, qui a été mis en place et paramétré pour filtrer les signaux parasites mesurés. Les bénéfices de ces diverses optimisations des paramètres de mesures ont été démontrés et ils ont permis de mesurer précisément des niveaux de SER très faibles. De plus, la mise en œuvre de la fabrication additive a permis de réaliser des analogues de géométrie maîtrisée avec des permittivités relatives locales à la carte ayant des parties réelles dans la gamme de 1 à 3 ; la permittivité étant ajustée par contrôle de la porosité. Les trois principales études présentées concernent : des sphéroïdes de faible permittivité, analogues de micro-algues, des agrégats de suies de forme complexes, et des scènes forestières composées d’analogues d’arbres et de véhicules. / This thesis widens the application of the microwave analogy by adopting the appropriate techniques to create objects of controlled shapes and electromagnetic properties using novel manufacturing technologies, as well as by developing a versatile setup providing accurate measurements. Microwave analogy is a useful approach to investigate a scattering problem when the targets have nanometric or metric sizes. The experiment is scaled to the microwave range and the target is mimicked by a centimeter-sized analog, while maintaining the same initial wavelength over target’s dimension ratio and conserving the same geometric and electromagnetic properties. A special attention is given to low scattering targets with RCS levels down to -60 dBm2. The random noise of the measurement setup was characterized and a novel optimization technique was proposed which consists of an angular decomposition of the bistatic region with different power profiles. The undesired reflections within the anechoic chamber were identified and a Hardgating system was installed, allowing to filter out the stray signals. In addition, additive manufacturing technologies were involved in the fabrication of analogs and a novel technique was proposed to obtain targets with “on-demand” shapes and local values of their complex permittivity. We can manufacture low scattering analogs of the real part of permittivity for any value between 1 and 3 by creating well controlled porous structures. The three main targets are studied: low permittivity spheroids, analogs of microalgae, soot aggregates analogs with complex shape, and scaled forest scene composed of tree analogs with some metal vehicle analogs.

Les matériaux absorbants des ondes électromagnétiques sont des éléments importants pour l'évaluation de nombreux systèmes électroniques militaires mais également civils. Ces matériaux sont utilisés, par exemple, pour la réduction des interférences électromagnétiques (EMI) dans divers composants sans fils, la réduction de la surface équivalente radar (SER) ou comme absorbants à l'intérieur des chambres de mesures. C’est cette dernière application qui est visée par les travaux de cette thèse. L’objectif de mes travaux de thèse est d’optimiser des matériaux absorbants utilisés dans les chambres anéchoïques. La géométrie et la composition du matériau absorbant sont les deux paramètres qui influencent la capacité d’absorption de l’onde électromagnétique par un matériau. Ce seront donc les deux pistes d’optimisation explorés durant cette thèse. Notre but est d’obtenir les absorbants présentant les plus faibles coefficients de réflexion et de transmission, soit une absorption élevée, ceci dans une large bande de fréquence. / The electromagnetic absorber materials are important elements for evaluating various electronic and civil systems. These materials are used, for example, for minimizing electromagnetic interferences (EMI) in different wireless components, for minimizing the radar cross section (RCS) or for usage in anechoic chambers. The latter application is the targeted work in this thesis. The objective of this work is to optimize the absorber materials used in anechoic chambers. The geometry and the material composition are the two parameters that influence the absorption of the electromagnetic wave inside the material itself. This are the two topics of optimization explored in this thesis. Our objective is to obtain material absorbers that have low reflection and transmission coefficients and high absorption in a large frequency band.

The master thesis summaries the findings results of a study focused on the spread of noise vibrating flat plates. Part of this thesis is measurements of flat plate noise in the anechoic chamber and finally their processing and evaluation. The next part of the thesis is focused on FEM acoustic simulation and comparing results with measurements.

No description available.

Durante la última década, los métodos de optimización heurísticos basados en imitar a nivel computacional procesos naturales, biológicos, sociales o culturales, han despertado el interés de la comunidad científica debido a su capacidad para explorar eficientemente espacios de soluciones multimodales y multidimensionales. En este ámbito, esta tesis aborda el desarrollo, análisis y puesta a punto de diferentes métodos de optimización tradicionales y heurísticos. En concreto, se considera un método de búsqueda local basado en símplex y varios métodos heurísticos, tales como el recocido simulado, los algoritmos genéticos y la optimización con enjambre de partículas. Para estos dos últimos algoritmos se investigan diferentes esquemas con el objetivo de superar las limitaciones propias de los esquemas clásicos.La puesta a punto de los diferentes métodos de optimización se realiza considerando como problema de referencia la caracterización de la radiación de antenas a partir de medidas en campo próximo sobre geometría plana, utilizando un método de transformación de campo cercano a campo lejano basado en corrientes equivalentes. Para cada método de optimización se incluye un análisis paramétrico, en los casos en los que se ha considerado necesario, así como resultados de transformación de campo teóricos obtenidos para diferentes antenas de apertura y antenas de bocina piramidal. Los resultados de un estudio comparativo, realizado utilizando fuentes teóricas y medidas, demuestran la utilidad del método y permiten concluir que la optimización con enjambre de partículas es el algoritmo que proporciona las mejores prestaciones para esta aplicación.Los métodos de optimización desarrollados e investigados en este trabajo han sido también aplicados a otros problemas como son la síntesis de agrupaciones lineales o el modelado de fuente en aplicaciones de compatibilidad electromagnética. / For the last decade, heuristic optimization methods based on imitating natural, biological, social or cultural processes in a computational way have aroused great interest among the scientific community, due to its ability to explore efficiently multimodal and high-dimension solution spaces. On this basis, this thesis tackles the development, analysis and tuning of different traditional and heuristic optimization methods. In short, a local search method based on simplex and several heuristic methods, such as simulated annealing, genetic algorithms and particle swarm optimization are considered. For these last two algorithms different schemes are investigated so as to overcome the typical limitations of classical schemes.The tuning of the optimization methods is carried out considering as a reference problem the antenna radiation characterization from near-field measurements over a planar geometry, using a near-field to far-field transformation method based on equivalent currents. A parametric analysis is included for each optimization method, in those cases in which it has been considered necessary, as well as theoretical field transformation results obtained with aperture and pyramidal horn antennas. Results of a comparative study, carried out using theoretical sources and measurements, demonstrate the usefulness of the method and make it possible to conclude that the particle swarm optimization is the algorithm that provides the best performance for this application.The optimization methods developed and investigated in this work have also been applied to other problems, such as the synthesis of linear arrays or the source modelling in electromagnetic compatibility applications.