Fresnel Zone Clearance Calculator
Work out how much clearance a point-to-point radio link needs around its line of sight. The first Fresnel zone is the football-shaped region carrying most of the signal energy; keeping at least 60% of it clear of terrain, trees and buildings avoids diffraction loss and keeps the link near free-space performance.
Enter Values
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How to use this calculator
- Enter the operating frequency f in GHz.
- Enter the total path length d between the two antennas in km.
- Enter d1, the distance from one end to the obstacle or point of interest in km; the tool derives d2 = d − d1 and returns the first Fresnel zone radius and the 60% minimum clearance.
How it works
The first Fresnel zone radius at a point along the path is r₁ = 17.32·√((d1·d2) / (f·d)), with d1, d2 and d in km, f in GHz and r₁ in metres. The zone is widest at the midpoint (d1 = d2). The usual engineering rule is to keep 60% of that radius (0.6·r₁) unobstructed, so both r₁ and the 60% clearance are reported.
Worked example
Worked example. For f = 2.4 GHz, d = 10 km and d1 = 5 km (mid-path): d2 = 5 km, so r₁ = 17.32·√((5 × 5) / (2.4 × 10)) = 17.32·√1.04167 = 17.677 m, and the 60% minimum clearance is 0.6 × 17.677 = 10.606 m.
Common mistakes
- Only clearing the visual line of sight — a link can have clear line of sight yet still lose signal because an obstacle intrudes into the Fresnel zone below it.
- Entering d1 equal to or greater than d: the obstacle must lie between the two ends, so 0 < d1 < d.
- Mixing units — distances must be in kilometres and frequency in GHz for the 17.32 constant to apply.
Frequently asked questions
Why is 60% clearance the target and not 100%?
Keeping 60% of the first Fresnel zone clear typically holds the extra diffraction loss to a fraction of a dB, which is a practical compromise between cost (antenna height, tower) and performance. Full clearance is ideal but rarely necessary.
Where is the Fresnel zone widest?
At the midpoint of the path, where d1 = d2. That is usually the critical point to check for obstructions, though a tall obstacle nearer one end can still block the narrower zone there.
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Tip: Enter any known values to calculate the remaining results.
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