Line of Sight and Terrain
Edit on GitHubHow mountains, ridges, and valleys shape the beacon's real range, with honest numbers for 433 MHz.
Line of Sight and Terrain
The 15 km figure in the marketing is line-of-sight over flat ground. Real terrain changes everything. This page gives the honest numbers.
The basic physics
433 MHz is a VHF-ish/UHF band: it needs a clear path. It does not follow the curvature of the earth and it does not bend around ridges. If a rock is between the antennas, there is no signal.
| Terrain | Typical range |
|---|---|
| Flat, clear line of sight, antennas raised | 5-15 km |
| Forest | 1-3 km (foliage loss) |
| Valley with a ridge blocking the path | 0.2-1 km |
| Across a valley, same ridge line (height match) | 3-10 km |
| Beacon buried in snow | 10-100 m depending on depth |
The mountain rule
In the Alps or any mountainous area, the practical rule is: you hear what you can see. If you can see the other side of the valley, the link likely works; if a ridge is in the way, it does not.
Height is everything
- A beacon at 2000 m talking to a base at 1800 m across a valley: great link.
- The same beacon 50 m lower, behind a shoulder: dead.
- Deploy on high points, put the antenna up, and orient the case so the antenna is vertical.
Foliage and snow
- Wet leaves absorb 433 MHz noticeably: expect 3-6 dB loss through a treeline.
- Snow has a moderate loss that grows with water content and depth; see the snow behavior page for numbers.
Planning around it
- Choose beacon positions with a visible path to where rescuers will be.
- Place the beacon high, not just “where the victim is”.
- During a search, the SEARCH team should climb to high points to get over blocking terrain.
- Know the terrain before you deploy; a map study beats radio hope.