Free RF calculator
Fresnel zone and earth bulge calculator
The Fresnel zone radius at any point on a line-of-sight path, the 60 percent clearance rule, and the earth bulge a long path adds. Give an obstruction's height relative to the direct ray and it adds the knife-edge diffraction loss.
Keep at least 60% of the first Fresnel zone clear. The clearance to hold is 0.6 x the radius plus the earth bulge; add the bulge to any obstruction survey height. Knife-edge loss uses the ITU-R P.526 approximation for a single edge.
Runs in your browser; nothing you enter is sent anywhere. The same math the Sentinel RF product uses, tested against it. A planning figure, not an engineering sign-off.
The math
r_n (m) = sqrt(n x lambda x d1 x d2 / (d1 + d2))
bulge (m) = d1 x d2 / (2 x k x Re)
v = h x sqrt(2 (d1 + d2) / (lambda x d1 x d2))
L (dB) = 6.9 + 20 log10(sqrt((v - 0.1)^2 + 1) + v - 0.1), v > -0.7
d1 and d2 are the distances to each end, Re is the earth's radius, and the loss line is the ITU-R P.526 approximation for a single knife edge.
Reading the result
The clearance to hold at a point is 60 percent of the first-zone radius plus the earth bulge there. A k factor of 4/3 is the standard atmosphere; designers check a smaller k, such as 2/3, for the conditions that bend the ray toward the ground.
A knife-edge loss near 6 dB means the edge is grazing the ray. Below the ray the loss falls toward zero; above it, it climbs quickly.
In Sentinel RF
The path study pulls the actual USGS elevation profile between two points, sampled about every 10 meters, and draws the direct ray, the first Fresnel zone and the earth bulge over it, so the tightest point on the path is found rather than guessed.
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The product runs these against real licenses, real towers and real terrain, and prints the result under your operation's name.