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 It is the mechanism behind long-distance shortwave broadcast, amateur DX, and much HF utility traffic, and it is what makes the HF bands feel alive and unpredictable compared with steady line-of-sight VHF.

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

How it works

Solar ultraviolet and X-rays strip electrons from atoms high in the atmosphere, creating ionised layers — the D, E, and F regions — between roughly 60 and 400 km up.² A radio wave entering this ionised gas is progressively refracted: the free electrons slow the wave’s upper part relative to its lower part, bending its path. At HF the bend can be sharp enough to turn the wave back toward Earth, so what looks like a reflection is really cumulative refraction. The wave returns to the surface far away, may bounce off the ground, and refract again — multi-hop paths can circle much of the planet.

Whether a given frequency skips depends on the electron density and the launch angle:

• Maximum usable frequency (MUF) — the highest frequency the ionosphere will bend back for a given path. Above the MUF the wave punches through into space instead of returning.
• Critical frequency — the MUF for a straight-up wave; a rough gauge of the layer’s strength.
• Skip zone — a ring where the ground wave has died out but the first hop hasn’t yet come down, so the signal is unheard.

In practice

Because ionisation is driven by the sun, skywave conditions change with the time of day (the D layer absorbs by day and fades at night, opening the low bands after dark), season, and the ~11-year solar cycle. The same 20 m signal that reaches across an ocean at noon may be gone by midnight, while 80 m does the opposite. Higher bands — VHF and up — carry too much frequency for the ionosphere to bend, so they normally pass straight through and stay line-of-sight, governed instead by the radio horizon. Below HF, the ground wave provides steady local coverage that does not depend on the ionosphere at all.

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. Ionospheric paths add fading, Doppler, and multi-path spreading that stress narrowband decoders, so HF digital modes are built to be robust against them. GopherTrunk’s focus is land-mobile VHF/UHF trunking, which lives above the skip bands, so ionospheric propagation is context for the wider spectrum rather than a path GopherTrunk itself decodes.

Sources

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

2. Ionosphere — Wikipedia, on the D/E/F layers, ionisation by solar radiation, and the MUF/critical frequency. ↩

See also

• Radio propagation
• Sky-wave propagation
• Ground-wave propagation
• Frequency bands (HF/VHF/UHF)
• Airspy HF+

Learn it step by step

• Frequency, bands & the spectrum