Amélioration du découplage inter-éléments par surface haute impédance pour des antennes réseaux GNSS compactes

L'objectif de cette thèse est de proposer une solution à base de Surface Haute Impédance (HIS) permettant de découpler les éléments rayonnants d'un réseau GNSS pour améliorer un système CRPA (Controlled Reception Pattern Antenna). Nous concevons alors une HIS possédant une taille de cellule élémentaire inférieure à lambda0/14 et caractérisons sa bande interdite électromagnétique (EBG). Un patch est choisi comme antenne élémentaire du réseau CRPA. Différentes configurations de mise en réseau incorporant des HIS idéales ou non sont étudiées pour affiner la conception de notre antenne élémentaire. Nous menons ensuite une étude détaillée des phénomènes résonants dans les HIS. Nous proposons alors l'utilisation de résonateurs originaux et compacts inspirés des HIS. Finalement, nous concevons et réalisons différentes antennes patchs en cavité environnées par des résonateurs. Des résultats de simulations et de mesures mettent en évidence le très fort potentiel de découplage de ces structures.This thesis addresses the problem of mutual coupling reduction between the elements of a compact GNSS array - using High Impedance Surfaces (HIS) - to improve Controlled Reception Pattern Antenna (CRPA) performances. We first design a HIS, which elementary cell size is smaller than lambda0/14. Its Electromagnetic Band Gap (EBG) properties are characterised numerically and experimentally. A patch antenna is chosen as the elementary antenna of the CRPA. Ideal HIS are proposed to study different array configurations in order to define the elementary antenna. Resonant mechanisms occurring at the frequencies of the band gap in the HIS are then studied in detail. As a result, we propose the use of new compact HIS like resonators. Finally, different prototypes are designed and built. They consist of cavity backed patch elements surrounded or not by the proposed HIS like resonators. Numerical and experimental results demonstrate the mutual coupling drastic reduction achieved with these structures

Antenna Gain and Link Budget for Waves Carrying Orbital Angular Momentum (OAM)

This paper addresses the RF link budget of a communication system using unusual waves carrying an orbital angular momentum (OAM) in order to clearly analyse the fundamental changes for telecommunication applications. The study is based on a typical configuration using circular array antennas to transmit and receive OAM waves. For any value of the OAM mode order, an original asymptotic formulation of the link budget is proposed in which equivalent antenna gains and free-space losses appear. The formulations are then validated with the results of a commercial electromagnetic simulation software. By this way, we also show how our formula can help to design a system capable of superimposing several channels on the same bandwidth and the same polarisation, based on the orthogonality of the OAM. Additional losses due to the use of this degree of freedom are notably clearly calculated to quantify the benefit and drawback according to the case.Comment: 33 pages, 11 figure

Full-Metal Reflectarray for Space Applications

This paper proposes a new full-metal unit cell for reflectarray antenna for space applications. It can be scaled to operate from X-band to Ka-band with the same manufacturing process. In this paper, An X-band 3-panel reflectarray for small satellite has been designed in this paper. The reflectarray has been simulated on the [8-8.4] GHz band and shows a good stability on the band. The maximum gain is 28 dB and the half-power beamwidth is 8°

Voxel-based assessments of treatment effects on longitudinal brain changes in the Multidomain Alzheimer Preventive Trial cohort

International audienceObjective: The Multidomain Alzheimer Preventive Trial (MAPT) was designed to assess the effect of omega-3 supplementation and a multidomain intervention (physical activity, cognitive training and nutritional advice) on cognitive decline of people with subjective memory complaint. In term of cognitive testing, no significant effect on cognitive decline was found over the 3-year follow-up. Yet, in the context of dementia-related conditions, brain morphological changes can be used to foretell the cognitive evolution. In this paper, we evaluate the effect of the interventions on the evolution of the brain morphology using the MR images acquired during MAPT. Methods: Subjects in the MAPT cohort with two MRI acquisitions, at baseline and at 36 months, were included , resulting in a subset of 376 subjects distributed in the 4 intervention groups: multidomain intervention plus omega-3, multidomain intervention plus placebo, omega-3 alone, and placebo alone. The morphological changes were assessed from volume measurements of regions of interest and a voxel-wise deformation-based approach. The primary outcome is the longitudinal deformation observed between the baseline image and the 3-year follow-up. Results: The multi-domain intervention is associated with a significant effect on the 3-year morphological evolution. The effect is similar within the two groups undergoing the intervention regardless of the omega-3 or placebo treatment. The voxel-wise deformation-based approach shows that the differences are mainly located in the left peri-ventricular area next to the temporoparietal junction (TPJ). These morphological changes correspond to a slower morphological evolution and are correlated with a better performance in cognitive assessments. These results could not be observed using the volumetric morphometry approach. No effect of omega-3 was observed.Discussion: In this study, we found that the multidomain intervention has a significant effect on morphological changes that are usually associated with the cognitive decline. This result suggests that effects at the level of cognitive decline may be visible in the long term, and that the cognitive scores may not be powerful enough to detect changes after 3 years. We argue that the use of neuroimaging could help define whether early intervention strategies are effective to delay cognitive decline and dementia

Eddy-resolving simulation of plankton ecosystem dynamics in the California Current System

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 53 (2006): 1483-1516, doi:10.1016/j.dsr.2006.06.005.We study the dynamics of the planktonic ecosystem in the coastal upwelling zone within the California Current System using a three-dimensional, eddy-resolving circulation model coupled to an ecosystem/biogeochemistry model. The physical model is based on the Regional Oceanic Modeling System (ROMS), configured at a resolution of 15 km for a domain covering the entire U.S. West Coast, with an embedded child grid covering the central California upwelling region at a resolution of 5 km. The model is forced with monthly mean boundary conditions at the open lateral boundaries as well as at the surface. The ecological/biogeochemical model is nitrogen based, includes single classes for phytoplankton and zooplankton, and considers two detrital pools with different sinking speeds. The model also explicitly simulates a variable chlorophyll-to-carbon ratio. Comparisons of model results with either remote sensing observations (AVHRR, SeaWiFS) or in situ measurements from the CalCOFI program indicate that our model is capable of replicating many of the large-scale, time averaged features of the coastal upwelling system. An exception is the underestimation of the chlorophyll levels in the northern part of the domain, perhaps because of the lack of short-term variations in the forcing from the atmosphere. Another shortcoming is that the modeled thermocline is too diffuse, and that the upward slope of the isolines toward the coast is too small. Detailed time-series comparisons with observations from Monterey Bay reveal similar agreements and discrepancies. We attribute the good agreement between the modeled and observed ecological properties in large part to the accuracy of the physical fields. In turn, many of the discrepancies can be traced back to our use of monthly mean forcing. Analysis of the ecosystem structure and dynamics reveal that the magnitude and pattern of phytoplankton biomass in the nearshore region are determined largely by the balance of growth and zooplankton grazing, while in the offshore region, growth is balanced by mortality. The latter appears to be inconsistent with in situ observations and is a result of our consideration of only one zooplankton size class (mesozooplankton), neglecting the importance of microzooplankton grazing in the offshore region. A comparison of the allocation of nitrogen into the different pools of the ecosystem in the 3-D results with those obtained from a box model configuration of the same ecosystem model reveals that only a few components of the ecosystem reach a local steady-state, i.e. where biological sources and sinks balance each other. The balances for the majority of the components are achieved by local biological source and sink terms balancing the net physical divergence, confirming the importance of the 3-D nature of circulation and mixing in a coastal upwelling system.Most of this work has been made possible by two grants from NASA. Additional support is acknowledged from NSF’s ITR program

Design of a dielectric resonator antenna with a customized dielectric constant based on zirconia

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A Dielectric Resonator Antenna designed with a structured dielectric material

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Scalar metasurface antennas with tilted beam

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