Tropospheric Ducting
Edit on GitHubThe weather phenomenon that carries 433 MHz signals far beyond normal range, and what to do when it happens.
Tropospheric Ducting
Every few weeks, in the right weather, a layer of warm air traps VHF and UHF signals and guides them for hundreds of kilometers. That is tropospheric ducting, and it changes the rules of the band.
The mechanism
Normally the atmosphere’s temperature falls with height. In a temperature inversion, warm air sits above cold air. Radio waves bend into the boundary and get trapped, traveling along the “duct” with very low loss. The effect is strongest for horizontally-polarized waves at VHF/UHF.
When it happens
- Clear nights after warm days (radiation inversions)
- High-pressure systems in summer and autumn
- Over seas and lakes, where moist air layers form
- Often in the early morning before the sun breaks the inversion
What you will notice
- A 433 MHz signal from 200 km away at S9, like it was next door
- Your beacon’s SEARCH mode showing strong signals on many channels
- PMR446 and ISM devices keying up that you have never heard before
What to do
- In SEARCH mode, pick the quietest channel, not the “loudest” one
- Do not assume a strong signal means a close transmitter
- If beaconing for a drill, consider moving to a less crowded channel or a different band segment
- Report ducting to your local radio club; it is a known, watchable phenomenon
The good side
Ducting can also work in your favor: a beacon on a ridgeline during an inversion might be heard by a rescue team 100 km away. Do not plan on it, but do not be surprised by it.
Related pages
- Propagation Modes for the other modes
- Interference Sources for who shares the band
- Frequency Planning Examples for channel choice