Fresnel Zone Radius
Calculate the first Fresnel zone radius, r = 17.31·√(d₁·d₂ ÷ (f·(d₁+d₂))), from the distances of each end to the point d₁ and d₂ (km) and the frequency f (GHz). The result, in metres, defines the ellipsoid around the line of sight that must stay clear of obstacles for a radio link to work without diffraction loss. Rule of thumb: at least 60% of the first Fresnel zone should be unobstructed. It is essential in designing point-to-point, microwave and long-range Wi-Fi links. Enter the distances and the frequency.
Result
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Raio da zona de Fresnel
Num enlace de rádio, o sinal não viaja apenas pela linha reta entre as antenas — ele ocupa um volume elipsoidal em torno dessa linha, e ondas que percorrem caminhos ligeiramente diferentes chegam com fases distintas. A primeira zona de Fresnel é o elipsoide onde essas ondas chegam praticamente em fase, reforçando o sinal direto; se um obstáculo (morro, prédio, árvore) invade essa zona, causa difração e perda, mesmo havendo linha de visada limpa. O raio máximo dessa zona (no ponto considerado) é r = 17,31·√(d₁·d₂ ÷ (f·(d₁+d₂))), com as distâncias d₁ e d₂ de cada antena ao ponto em km, a frequência em GHz e o raio em metros. O raio é maior no meio do percurso e em frequências baixas. A regra prática de projeto é manter pelo menos 60% da primeira zona de Fresnel desobstruída para que a perda por difração seja desprezível — por isso enlaces longos exigem antenas altas, que elevam a linha de visada acima dos obstáculos. Informe as duas distâncias e a frequência.
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