Also known as: ionospheric propagation, skywave

Ionospheric propagation (skywave) is the refraction of HF radio waves by ionised layers of the upper atmosphere, allowing signals to “skip” over the horizon for hundreds or thousands of kilometres.¹

HF signals can refract off the ionosphere and "skip" thousands of kilometres beyond the horizon.

How it works

The ionosphere’s electron density varies with the sun, so HF skip changes with time of day, season, and the solar cycle. Higher bands (VHF and up) generally pass through the ionosphere rather than reflecting, so they stay line-of-sight.

Relevance to SDR

Receiving HF skip needs an HF-capable radio such as the Airspy HF+ or an upconverter, since a basic RTL-SDR does not tune HF directly.

Sources

1. Skywave — Wikipedia, on ionospheric refraction of HF radio waves and long-distance skip propagation. ↩

See also

• Radio propagation
• Frequency bands (HF/VHF/UHF)
• Airspy HF+

Learn it step by step

• Frequency, bands & the spectrum