Link planning

How to Calculate Fresnel Zone Clearance for a Wireless Link

A link can have clear line-of-sight on a map and still perform badly if an obstruction sits inside its Fresnel zone. Here's the formula and how to apply it before you plan a new tower-to-customer or tower-to-tower link.

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What the Fresnel zone actually is

Radio signal doesn't travel in a perfectly straight pencil beam between two antennas — it propagates in a cigar-shaped volume around the direct line of sight. The first Fresnel zone is the innermost of these volumes; an obstruction that intrudes into it (a tree, a rooftop, rising terrain) can attenuate the signal even when you can technically “see” the other end.

The formula

The radius of the first Fresnel zone at any point along a link, in meters, is approximately:

r = 17.3 × √( (d1 × d2) / (f × d) )

r  = Fresnel zone radius, in meters
d1 = distance from one end to the point of interest, km
d2 = distance from the other end to that point, km
d  = total link distance, km  (d1 + d2)
f  = frequency, GHz

The radius is largest at the midpoint of the link (where d1 = d2 = d/2), which is usually the point worth checking first for a quick estimate.

Worked example

A 10km backhaul link at 5.8GHz. At the midpoint, d1 = d2 = 5, d = 10, f = 5.8:

r = 17.3 × √( (5 × 5) / (5.8 × 10) )
r = 17.3 × √( 25 / 58 )
r = 17.3 × 0.657
r ≈ 11.4 meters

That's the radius of the first Fresnel zone at the link's midpoint — a cylinder you want substantially clear of obstructions, centered on the direct line between the two antennas.

The clearance rule of thumb

You don't need 100% clearance of the first Fresnel zone to get a usable link, but the common field rule is to keep 60–80% of the first zone clear of any obstruction along the entire path, not just the midpoint — check it at several points, especially near tall obstructions close to either end, since the zone narrows quickly near the antennas.

What this estimate doesn't cover

This formula alone has no terrain or elevation data — it tells you the zone's geometry, not whether a specific hill or building actually sits inside it. Treat a Fresnel estimate as a fast first pass that tells you whether a link is even worth a real survey, not a replacement for one. For an existing tower network, a planning tool that already has every tower's coordinates can at least draw the midpoint estimate on the map automatically — TowerLens's 📐 Plan link panel does exactly this between any two existing devices or a proposed new point.