The Fresnel Zone
Edit on GitHubWhy line-of-sight is not enough, how the Fresnel zone blocks signal, and what to do about it in the mountains.
The Fresnel Zone
Radio waves do not travel on a razor-thin line. They spread into a football-shaped region between transmitter and receiver called the Fresnel zone, and anything poking into it weakens the signal.
The concept
Between two antennas there is a clear “tube” of space where the signal travels. For a link to work at full strength, that tube should be free of obstacles. The zone’s radius at the midpoint is:
r ≈ 8.66 m × sqrt(km / GHz)At 433 MHz (0.433 GHz) over 1 km: r ≈ 8.66 × sqrt(1 / 0.433) ≈ 13 m. Over 10 km: r ≈ 41 m.
What this means for the beacon
- A ridge or building that is not in the direct line can still block the signal if it reaches into the zone.
- Over short rescue ranges (hundreds of meters), the zone is small and less of a problem.
- Over multi-kilometer links, the antennas should be raised so the midpoint clears obstacles.
Practical mountain rules
- Get height: the higher the antenna, the bigger the clear zone below it.
- A beacon on the ground in a valley has the terrain in its zone on almost every path.
- Deploy the beacon on a high point when possible; a 1-2 m pole or a branch beats lying on the ground by a huge margin.
- Do not aim for a specific direction; the beacon radiates omnidirectionally, so height is the lever, not pointing.
The good news
For the beacon’s real job - being found within a kilometer or two - the zone is small and rarely the deciding factor. Terrain line-of-sight matters far more. The Fresnel zone matters when you are planning a 5+ km link between a beacon on a peak and a base station.
When to ignore it
Short-range bench tests and searches within ~500 m: ignore the zone, focus on placement and height.