Mountain Effects on Radio
Edit on GitHubHow mountains shape 433 MHz propagation: ridge shadowing, valley trapping, knife-edge diffraction, and the tactics that exploit them.
Mountain Effects on Radio
Mountains are the beacon’s home turf, and they are also its worst enemy. This page is the tactical manual for radio in relief.
Ridge shadowing
A ridge between beacon and receiver blocks the direct path. The shadow can cost 20 - 40 dB, effectively ending the link. The signal does not fully disappear: diffraction over the ridge edge leaks some energy through, which is why a beacon “sometimes” works from behind a ridge.
Knife-edge diffraction
When the path just clips a ridge top, the edge acts as a diffracting knife edge and the loss is surprisingly modest (10 - 15 dB). This is why a beacon directly below a summit may be heard from the other side while one deeper in a valley is not: the geometry of the grazing edge matters more than the distance.
Valley trapping
A deep valley confines the signal; reflections off the walls add, but the direct path is gone. In a narrow valley the effective range can be a few hundred meters regardless of power. The classic fix is to climb out of the valley floor until the walls clear the path.
The tactical playbook
| Situation | Move |
|---|---|
| Beacon in valley, you on the slope | Ascend until you see the beacon’s ridge |
| Ridge between you and the beacon | Climb to the ridge crest; diffraction doubles |
| Search in a valley | Check the crest first, then the floor |
| Deploying a beacon | High open slope, not the valley floor |
The 45-degree rule
As a rule of thumb, if you can see the point where the beacon is deployed (even if the beacon itself is hidden), the path usually works. If a ridge blocks the view, assume it blocks the radio too, and plan the search to cross ridges.
Related pages
- Line of Sight and Terrain for the general rule
- Fresnel Zone for the clearance math
- Search Patterns and Procedure for the search side