Fresnel Zone Calculator

A Fresnel zone is a series of ellipsoidal regions around the direct line of sight between transmitter and receiver. Radio waves reflecting off objects inside these zones can strengthen or weaken the main signal. The first zone (n=1) is the most important for radio link planning.

R_n = √(n·λ·d₁·d₂/(d₁+d₂)), where n is the zone number (1, 2, 3, ...), λ is the wavelength in meters, d₁ is the distance from the transmitter to the point under review in meters, and d₂ is the distance from that point to the receiver in meters. The result R_n is in meters.

At the link midpoint (d₁=d₂=2.5 km), wavelength λ=5.17 cm, so: R₁=8.04 m, R₂=11.37 m, R₃=13.92 m. The minimum clearance under the practical 60% rule is 4.82 m.

A practical rule of thumb in radio link planning: at least 60% of the first Fresnel zone radius must be free of obstacles to keep extra attenuation small. This is practical guidance, not a formal standard — terrain conditions, the refraction K-factor, and fade margin still need consideration.

At the link midpoint (d₁=d₂), because the product d₁·d₂/(d₁+d₂) reaches its maximum when d₁=d₂. Near either end, the zone radius shrinks toward zero.

For a symmetric link (d₁=d₂=D/2), the inverse formula is D=4·R²/(n·λ). Example: to keep R₁ under 8 m at 5.8 GHz (λ=5.17 cm), the maximum total distance D≈4.95 km. Use the "Distance Solver" tab on the calculator above.