In this research, Fresnel zone plates were studied using an accurate, full-electromagnetic simulator based on the body-of-revolution finite-difference time-domain (BOR-FDTD) method. This tool was used to investigate zone plates for two different applications: zone plates used as antennas in communication systems and zone plates used as focusing elements in free-space, focused-beam measurement systems. Through detailed studies of zone plates for these applications, a number of general characteristics of the zone plate were given a more in-depth look than they have been given in the past. For the first application, parametric studies were performed for Soret, folded, and grooved-dielectric, phase-correcting zone plates for antenna applications. These studies were used to generate new design graphs, from which general observations are made about the effect of varying different parameters on the focusing ability of a zone plate. For the grooved-dielectric, phase-correcting zone plate, these studies show that a number of factors influence the focusing ability in ways that are unexpected, based on many of the simple techniques typically used to analyze zone plates. Each of these factors is evaluated individually. For the second application, a zone plate was designed to be used as a focusing element in a free-space, focused-beam measurement system. To determine the suitability of zone plates for this application, simulations and measurements of the electric field were used to compare this zone plate to a doubly-hyperbolic lens. A complete focused-beam system, using two zone plates as focusing elements, was used to measure the permittivity of different dielectric materials. These measurements are compared to results for a system that uses doubly-hyperbolic lenses and to published values. A technique for increasing the usable bandwidth of a zone plate in a focused-beam system was proposed and tested. Finally, as a topic for future research, the question is posed: Using the BOR-FDTD method, can a similar volume of material be shaped in a way that results in improved focusing performance compared to a traditional zone plate? Some insight into this open-ended question can be gained by examining preliminary results of the optimization of zone plates using of a genetic algorithm.

Une onde électromagnétique est définie par son amplitude, son vecteur d'onde, sa fréquence et son moment angulaire. Ce dernier peut être séparé en deux parties : la polarisation (associée au moment angulaire de spin), et le moment angulaire orbital. Ce dernier n'a vraiment été étudié qu'à partir de ces dernières années. Cette thèse a pour but d'approfondir les connaissances sur ce moment angulaire orbital. Des prototypes ont ainsi été réalisés, et des outils d'analyse ont été développés. Dans un premier temps, une étude du transfert de ce moment angulaire orbital à un objet macroscopique, à la fréquence de 870 MHz, est présentée. Une interprétation du mécanisme de transfert est ensuite proposée, supportée par le calcul des équations de champs et des simulations électromagnétiques. Dans un second temps, la conception et la réalisation de deux antennes, pouvant générer une onde possédant un moment angulaire orbital, sont présentées. Ces deux antennes utilisent une lame de phase avec une loi à dépendance angulaire. La première est une lame de phase dite « spirale », à permittivité constante et à hauteur variable. La seconde est une lame à gradient d'indice, à permittivité variable et à hauteur constante. Ces deux antennes ont été simulées, puis mesurées au sein de la chambre anéchoïque CHEOPS (DGA-MI, Bruz). Des cartographies sur un plan du champ ont ainsi été obtenues. Dans un dernier temps, la réflexion sur des surfaces courbes et planes, et l'influence de l'échantillonnage sur l'estimation des modes d'OAM, ont été étudiées. En ce qui concerne la réflexion, les résultats de simulations sont prometteurs, et semblent indiquer l'existence d'une relation entre les déformations du faisceau réfléchi et le type de surface. Pour l'estimation des modes d'OAM, les valeurs de champs sont extraites sur un cercle. Différents paramètres (positionnement et rayon du cercle) ont été considérés, et une méthode d'estimation des modes d'OAM sur une large bande de fréquence est proposée. / An electromagnetic wave is defined by its amplitude, its wave vector, its frequency and its angular momentum. This momentum can be decomposed into two components: polarization (associated to spin angular momentum), and orbital angular momentum. The later has not been investigated thoroughly until the last few years. To deepen the knowledge on orbital angular momentum, both prototypes and analysis tools have been developed in this thesis. First, study of orbital angular momentum transfer to a macroscopic object, at a frequency of 870 MHz, is presented. An interpretation of the transfer mechanism is then proposed, supported by the calculation of field equations and electromagnetic simulations. Secondly, the conception and the realization of two antennas able to generate an EM wave with orbital angular momentum are presented. These antennas use phase plates with an angular dependent law. The first one is a “spiral” phase plate, with constant permittivity and variable height. The second one has variable permittivity and constant height. Both have been simulated and characterized in CHEOPS anechoic chamber (DGA-MI, Bruz). Measurements of magnitude and phase, on a plane, have been obtained. Lastly, total reflection on planar and curved surfaces and influence of sampling on OAM modes estimation were investigated. Simulation results of reflected waves are promising and show a relationship between the induced deformations and the object reflected on. As for OAM modes estimation, study has been performed using an extraction circle on the wave front. Several parameters (center of the circle, radius) have been considered, and a method for OAM modes estimation on a large bandwidth is proposed.

The new and forthcoming generation of mobile networks intend to operate at considerably higher frequencies than the previous systems. This lift in frequency of operation alleviates today’s communication systems’ crowded bandwidth and allows for faster data rates than previously possible. However, the suggested increase in frequency of operation introduces new challenges and new antenna solutions are required. One possible candidate for the future communication systems is the Luneburg lens antenna that offers high gain, a simple feeding network and wide-angle scanning. Scanning of lens antennas occurs by placing several feeds along its focal line, but where the width size of the feed place a major constraint on the achievable crossover level between beams. In this thesis, we aim to increase the crossover level between beams in a geodesic Luneburg lens antenna. The importance of a high crossover level is to ensure a more equal performance in terms of data rate transfer to all end users. Here, we investigate two different methods on achieving a higher crossover level. The first method is to utilize a near-field lens while the other method concerns the usage of a generalized Luneburg lens that allows to displace the focal point outside the lens’ contour. A comparison study of these two alternatives are made where it is shown that a generalized Luneburg lens is the preferable choice. A generalized geodesic Luneburg lens is thereafter designed that attains a crossover level of -3:87 dB at the central frequency 62 GHz for the center port. The lens performs well with a bandwidth of 15% and a scanning range between ±52°. The reflection coefficient is below -13 dB in the frequency range of interest and the cross-talk is below -17:9 dB. The realized gain is simulated to 19:01 dBi at 57 GHz, 20:85 dBi at 62 GHz and 21:34 dBi at 67 GHz for the central port. / Den nya och de kommande generationerna av mobilnät avser att fungera på betydande högre frekvenser än tidigare system. Det här lyftet i frekvens minskar den trånga bandbredden i dagens kommunikationssytem och tillåter för snabbare datahastigheter än tidigare möjligt. Däremot, den förslagna ökningen av frekvens introducerar nya utmaningar och därmed behövs nya antennlösningar. En möjlig kandidat för det framtida kommunikationssystemet är Luneburg linsantennen som erbjuder en hög antennförstärkning, ett enkelt matningsnätverk och en bred vinkelskanning. Vinkelskanning av linsantenner sker genom att placera flera matningar längs dess fokallinje, men där bredden på matningarna utgör en stor begränsning för den nåbara korsningsnivån mellan strålningsfält. Det här examensarbetets syfte är att öka korsningsnivåerna mellan strålningsfälten i en geodetisk Luneburg linsantenn. Betydelsen att ha höga korsningsnivåer mellan strålningsfält är att säkerhetsställa en mer jämn prestanda av datahastigheter för alla slutanvändare. Vi undersöker två olika metoder för att uppnå högre korsningsnivåer. Den första metoden använder en närfältslins medan den andra metoden använder sig av en generaliserad Luneburg lins som tillåter att förflytta fokalpunkten utanför linsens kontur. En jämförelsestudie mellan dessa två metoder är genomförd där det visas att den generaliserade Luneburg linsen är det fördelsaktiga valet. En generaliserad Luneburg lins är därefter designad som uppnår korsningsnivåer på -3:87 dB på den centrala frekvensen 62 GHz för center porten. Linsen fungerar väl med en bandbredd på 15% och vinkelskanning mellan ±52°. Reflektionskoefficienten är under -13 dB i frekvensområdet av intresse och kopplingen mellan olika portar är under -17:9 dB. Den realiserade antennförstärkningen är simulerad till 19:01 dBi vid 57 GHz, 20:85 dBi vid 62 GHz och 21:34 dBi vid 67 GHz.