HF propagation and space weather
Why does 20 m work by day, 80 m at night and 10 m sometimes not at all? HF propagation depends on the ionosphere, and that depends on the sun. The page explains the relationships and shows current space weather indices.
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What the page does
- Ionospheric layers and their effect
- MUF and critical frequency
- Daily, seasonal and solar cycle variation
- Solar activity, K-index and aurora
- Rules of thumb for band choice
Background
The F layer of the ionosphere reflects HF back to earth. How high the frequency may be is given by the MUF (maximum usable frequency); it rises with solar radiation. During the day the lower D layer absorbs the lower bands; at night it disappears. Geomagnetic storms, indicated by a high K-index, mainly disturb paths across the polar regions.
In practice
How it works: first look at the current space weather indices and band conditions. As a rough rule, the higher bands from 20 m upwards work by day, the lower bands from 40 m downwards at night. When the K-index rises, paths to the south work better than across the pole. The actual situation is best shown by listening to a band with beacons or active stations.
Frequently asked questions
Where does the space weather data come from?
From public space weather services; the sources are listed in the app.
What does a K-index of 5 mean?
A geomagnetic storm. HF propagation is often disturbed, and aurora becomes possible in Central Europe.