Fresnel zone and terrain profile
Line of sight alone is not enough: a radio link needs clear space around the line of sight, the first Fresnel zone. The calculator shows its radius along the path and checks whether terrain and earth curvature block it.
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What the page does
- Radius of the first Fresnel zone at every point
- Terrain profile with earth curvature
- Check for 60 % clearance as a rule of thumb
- Suitable for microwave links, repeater sites and LoRa
- Worked steps with formula
Background
The radius of the first Fresnel zone is r = 17.32 · √(d₁ · d₂ / (f · d)) m, with d₁ and d₂ as distances to both ends in km, d as path length in km and f in GHz. In the middle of a 10 km path at 2.4 GHz that is about 17.7 m. As a rule of thumb at least 60 % of this radius should be free of obstacles.
In practice
How it works: set both end points on the map and enter antenna heights and frequency. The terrain profile shows the elevation between the points, the line of sight and the Fresnel zone. Where the terrain reaches into the zone, the link is at risk. Higher antennas at one or both ends, or an intermediate site, usually help.
Frequently asked questions
What happens if the Fresnel zone is blocked?
The signal is attenuated further, even if the other station is still visible.
Does this apply to LoRa?
Yes. Especially with low antennas on 868 MHz the Fresnel zone is often the limiting factor.