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QU’EST-CE QUE LA RADIO ?
1 . QU'EST CE QUE LA RADIO : CULTURE GENERALE
Un signal radio est une onde électromagnétique qui se déplace à la vitesse de la lumière. Correctement codé, ce signal peut transporter de l’information.
C’est James Clerk Maxwell qui découvrit le premier que la variation d’un champ magnétique induit un champ électrique qui induit à son tour un changement de champ magnétique et que la transition produit une onde électromagnétique. Cette découverte fut mise en pratique par Heinrich Hertz et surtout par Édouard Branly qui découvrit comment les détecter. Hertz avait fait l’émetteur, Branly le récepteur. Il ne restait qu’à inventer l’antenne, ce dont se chargea un dénommé Popov. C’est Marconi qui assembla les pièces du puzzle pour réaliser la première communication radio.
L’invention de Branly se limitant à la détection d’une onde en tout ou rien, les premières communications étaient en morse sur le mode transmission/non transmission.
Un peu plus tard, on découvrit la modulation qui permit alors de transporter un signal audio sur la radio, donc de la phonie, puis de la vidéo.
2. LA TRANSMISSION RADIO
2.1. PRINCIPE D’UNE LIAISON RADIO
Pour établir une telle liaison il faut un émetteur et un récepteur.
L’émetteur reçoit un signal électrique qu’il convertit en signal électromagnétique, lequel est conduit vers l’antenne.
L’antenne émet le signal… dans la nature… où il peut être capté par une ou plusieurs autres antennes. Ceci constitue une différence fondamentale avec les liaisons filaires pour lesquelles un signal ne peut être reçu que par celui qui se trouve au bout du fil. En radio, le signal est diffusé partout et est donc susceptible d’être reçu par tout le monde.
L’antenne réceptrice conduit le signal reçu vers le récepteur d’où est extrait le même signal électrique que celui qui était en entrée de l’émetteur.
Principe d’émission – réception
Tout ceci mérite quelques commentaires :
- L’onde émise va s’atténuer, d’autant plus que la distance entre l’émetteur et le récepteur est grande. De ce fait, le signal électromagnétique reçu par l’antenne est très faible et doit être ré-amplifié au niveau du récepteur.
- L’onde radio transmise entre l’émetteur et le récepteur peut être soumise à des perturbations qui vont l’altérer. Les éléments perturbateurs peuvent être d’autres émetteurs radio, mais aussi des appareils électriques apparemment anodins, mais qui génèrent un rayonnement électromagnétique. C’est le cas notamment des appareils qui utilisent un moteur.
- L’onde radio peut rencontrer des obstacles qui vont nuire à sa propagation entre l’émetteur et le récepteur. Ces obstacles peuvent être des bâtiments, des forêts… Certains obstacles, tels les parois métalliques, sont des obstacles absolus au passage des ondes électromagnétiques.
- Les ondes radio se propagent en ligne droite, mais elles peuvent rebondir sur des obstacles, une fois, plusieurs fois… Ainsi, le signal émis pourra-t-il emprunter simultanément divers trajets et arriver en plusieurs exemplaires au destinataire. Ces différents exemplaires se brouillent mutuellement et de trop grandes différences de délai de transmission entre plusieurs signaux reçus vont nuire à la restitution du signal transmis.Pour mettre toutes le chances de son côté, il faudra prendre certaines précautions :
- Installation de l’antenne en hauteur car plus elle est haute, plus la portée est grande. De plus,l’installation en hauteur permet d’éviter certains obstacles.
- Blinder les câbles qui conduisent le signal à l’antenne et éviter de le poser à côté de sources importantes de parasites, en particulier les ascenseurs.
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