Fenômeno Eletromagnético (Parte 3)
Autor: carlos ibarra | Publicado: 2011-09-23T10:28:00-07:00


&nbsp;As ondas de rádio, um fenômeno eletromagnético, são a base das telecomunicações sem fio. Uma onda de rádio é caracterizada por sua variação na unidade do tempo ou frequência. Associado ao conceito de frequência está o de onda. A onda se caracteriza pela distância percorrida por uma oscilação, ao efetuar um ciclo completo. Outra característica da oscilação é a sua amplitude, medida pelo ponto máximo alcançado na sua variação&nbsp;
Em resumo: frequência, amplitude e comprimento de onda caracterizam uma oscilação eletromagnética. Toda oscilação eletromagnética, qualquer que seja se move (no vácuo) com a velocidade da luz. A velocidade da luz (300 mil km/s) faz parecer que uma telecomunicação é instantânea, mas nem sempre. Satélites que estão muito distante introduzem um pequeno retardo na comunicação, por exemplo. Quanto menor o comprimento de onda (mais alta a frequência) há mais há interação com o meio físico. A partir de certo comprimento de onda, vale dizer de certa frequência, até uma gota de chuva pode ser um obstáculo (ao absorver a energia do sinal).&nbsp;
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As oscilações eletromagnéticas, como as ondas de rádio, são atenuadas pelo meio de propagação. Quanto maior a distância entre emissor e receptor mais o sinal se atenua (a mesma energia inicial do sinal que se expande precisa ocupar volumes crescentes). Conforme sua frequência, a oscilação eletromagnética se propaga de maneira diferente no planeta. Ondas extra-longas são utilizadas para comunicações mundiais (no fundo do mar). Ondas longas e médias são denominadas ondas terrestres e são utilizadas em broadcasting (radiodifusão). Ondas curtas utilizam reflexões múltiplas entre solo e ionosfera e dão cobertura, sem fio, ao mundo todo. Microondas são utilizadas em “visada direta” e vão pouco mais além do horizonte (daí antenas ficarem em pontos altos, inclusive satélites). 
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