Antenne Yagi

Résumé

thumb|Antennes Yagi thumb|Groupe de 8 antennes Yagi pour la bande EME L'antenne Yagi ou antenne Yagi-Uda (du nom de ses inventeurs, Hidetsugu Yagi et Shintaro Uda) est une antenne à éléments parasites utilisable des HF aux UHF. Mécaniquement simple à réaliser, elle est très utilisée en télévision terrestre, en liaisons point à point et par les radioamateurs. Elle fut inventée peu après la Première Guerre mondiale et utilisée pour les premiers radars. Elle est souvent appelée antenne râteau car les anciens modèles ressemblaient à un râteau. Principe de fonctionnement Une antenne Yagi peut être assimilée à une antenne réseau dont les éléments seraient alimentés par induction mutuelle. Si les espacements et longueurs des brins sont optimaux, le diagramme de rayonnement et le gain est celui d'un réseau. Une autre image simplifiée est celle d'une focalisation : l'ensemble des éléments parasites se comporte comme une lentille diélectrique. On peut démontrer que les propriétés

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https://fr.wikipedia.org/wiki/Antenne_Yagi

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Chargement

GaAs nanowires: from doping to plasmonic hybrid devices

Alberto Casadei

Semiconductor nanowires (NWs) are filamentary crystals with the diameter ranging from few tens up to few hundreds of nanometers. In the last 20 years, they have been intensively studied for the prospects that their unique quasi-one dimensional shape offers to both fundamental and applied science. More recently particular attention has been dedicated to use NWs as building blocks for nano-electronic devices. In this thesis we investigate the electro-optical properties of NWs in order to put some light on the mechanisms governing the electrical transport and the light coupling between NWs and metal nanostructures. We investigate Be and C doping in GaAs NWs synthesized by Molecular Beam Epitaxy (MBE). We obtain a doping control over a large range of densities and we identify a new in situ incorporation path. Since strong surface impurity scattering in III-V materials degrade the electronic performances, we grew NWs passivated with an AlGaAs layer and we investigate their properties. The NW passivation allows for the increase of the electron mean free path by a factor of almost 10. In addition, we designed AlGaAs/GaAs modulation doping NWs. The modulation doping structure allows for the to enhancement of the NW electron mobility revealing excellent properties for the realization of nano-electronic devices. We calculate the electron distribution in the modulation doped NWs and we observe a six-fold symmetry with six 1D electron channels when the carrier concentration is high, while for low concentrations, electrons are delocalized in the GaAs NW core. Thanks to their special interaction with light, semiconductor NWs have opened new avenues in photonics, quantum optics and solar energy harvesting. Here, we design a new system composed of a NW and an array of nanoantennas. Initially, we successfully demonstrated the plasmonic coupling between NWs and nanoantennas, observing an electric field enhancement in the NW as a function of the nanoantenna's gap distance. This finding represented an initial step toward the development of coupled nano-structures for the realization of a new generation of solar cells, detectors and non linear optical devices. Near field coupling was also used between a NW and Yagi-Uda antennas to obtain directional emission. In particular the precise tuning of the Yagi-Uda dimensions and positions leads a strong variation of the NW emission, being able to change this from backward to forward. One of the major challenges for NWs full technological deployment has been their strong polarization dependence in light absorption and emission. Here, we demonstrate that a hybrid structure formed by GaAs NWs with a highly dense array of bow-tie antennas is able to modify the polarization response of a NW. As a result, the increase in light absorption for transverse polarized light changes the NW polarization response, including the inversion of the polarization response. We calculated that the absorption of transverse polarized light can be enhanced up to 15 times. We also fabricate several electrical devices proving our calculated predictions.

EPFL2016

Chargement

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We demonstrate the directional emission of individual GaAs nanowires by coupling this emission to Yagi-Uda optical antennas. In particular, we have replaced the resonant metallic feed element of the nanoantenna by an individual nanowire and measured with the microscope the photoluminescence of the hybrid structure as a function of the emission angle by imaging the back focal plane of the objective. The precise tuning of the dimensions of the metallic elements of the nanoantenna leads to a strong variation of the directionality of the emission, being able to change this emission from backward to forward. We explain the mechanism leading to this directional emission by finite difference time domain simulations of the scattering efficiency of the antenna elements. These results cast the first step toward the realization of electrically driven optical Yagi-Uda antenna emitters based on semiconductors nanowires.

American Chemical Society2015

Chargement

MIMO techniques allow remarkable improvements in the reliability and/or transmission rate of wireless communication systems. However, there are several major challenges towards the implementation of conventional MIMO concept in terminals with size, cost, and power constraints. Firstly, insufficient space impedes the design of efficient and decorrelated MIMO antennas. Second, MIMO traditionally demands each antenna to be fed by its own RF chain, which in turn results in greater hardware complexity, larger power consumption, and higher implementation cost. Among all reduced-complexity and antenna-decoupling schemes proposed so far, the so-called beam-space MIMO has attracted a great deal of interest as a potential solution for addressing both problems concurrently. The key idea therein is to engineer the radiation pattern of a single-feed antenna structure for each symbol period, such that multiple independent symbols directly modulate a predefined set of orthogonal virtual patterns in the far-field, therefore allowing true MIMO transmission using a single RF chain and a compact antenna structure. More important in practice, the transmitted information can be retrieved using a conventional MIMO receiver. However, the transformation of this idea into reality entails dealing with various practical aspects that are commonly overlooked in theoretical and conceptual developments. This dissertation explores the beam-space MIMO concept from the perspective of the antenna engineering, and aims at addressing several key issues associated with the actual design and implementation of beam-space MIMO systems. The early developments of beam-space MIMO concerned switched parasitic arrays. However, the requirement of utilizing several physically-separate radiators is inconvenient for practicable implementation in compact portable devices. To solve this problem, a single-radiator load-modulated antenna solution is proposed in this dissertation. Another primary challenge consists in emulating high-order modulation schemes such as PSK with realistic hardware. Here, an efficient beam-space MIMO strategy is developed, which allows transmitting PSK data streams of any modulation order using only purely reactive reconfigurable loads, and without the need for a symbol-rate dynamic matching network. The approach is illustrated by the design and fabrication of a realistic antenna for QPSK signaling. The performance of a beam-space MIMO system which utilizes the fabricated antenna is then investigated through over-the-air experiments, and compared with conventional MIMO in realistic environments. Embedding information in the radiation patterns, beam-space MIMO systems are expected to be inherently prone to multiplexing performance degradation in the presence of external field perturbation. This makes the study of near-field interaction influence on beam-space MIMO distinct from those carried out for the case of conventional systems. This issue is considered for the first time in this dissertation. Moreover, like any reconfigurable system, a beam-space MIMO system may suffer from bandwidth expansion of the transmitted signals. The final part of the work is directed towards this important issue. To reduce out-of-band radiation effect, a solution based on shaping the time-domain response of the reconfigurable components is presented. The studies presented in this thesis constitute a crucial step towards MIMO with simpler and cheaper hardware for real-life terminals.

EPFL2015

Chargement

GaAs nanowires: from doping to plasmonic hybrid devices

Alberto Casadei

EPFL2016

EPFL2015