RF · path clearance
Fresnel zone calculator.
How wide the zone around the line of sight is at an obstacle, and the 60 percent clearance to keep.
Inputs
Clearance
Enter values to calculate.
How do I calculate the Fresnel zone radius?
The radius of the n-th Fresnel zone at a point is r = sqrt(n x lambda x d1 x d2 / (d1 + d2)), where lambda is the wavelength and d1 and d2 are the distances from the point to each antenna (ITU-R P.526). Link designers commonly keep at least 60 percent of the first zone clear of obstructions; this 60 percent figure is a design rule of thumb, not a legal limit.
Worked example. At 2.4 GHz on a 10 km path, at the midpoint (5 km from each end), the first zone radius is 17.7 m, so 60 percent is 10.6 m of clearance above and around the line of sight. The radius is largest at the midpoint and shrinks toward each antenna.
This does not include earth curvature or antenna height; on long paths, add the earth bulge separately.
Frequently asked questions
What is the first Fresnel zone?
The ellipsoid of space around the line of sight within which the path length differs from the direct path by at most half a wavelength.
What is 60% Fresnel clearance?
A common design target of keeping 60 percent of the first zone radius free of obstructions.
Where is the Fresnel zone widest?
At the midpoint between the antennas.
Does this include earth curvature?
No. It is the zone radius only.
Sources
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The 60 percent clearance is a common design practice, not an ITU limit. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Geometry estimate. Use a terrain-profile survey or a propagation tool for a real link design.