Also known as: sky wave, skywave
Sky-wave propagation is long-distance HF communication in which a radio wave is refracted back to earth by the ionosphere, then reflected upward again by the ground, repeating in a series of “hops” that can span oceans.1 It is the same physical mechanism described under ionospheric propagation, viewed as a hop-by-hop geometry that determines how far a signal reaches and where it lands.
How it works
The ionosphere is a set of charged layers (D, E, and the F1/F2 layers) created when solar ultraviolet and X-rays ionise the thin upper atmosphere. A wave entering these layers at a shallow angle is progressively bent until it returns to earth. The steeper the launch angle, the higher the frequency the layer can still turn back; a wave that is too high in frequency or too steep simply punches through into space.
Two frequencies bound any given path:
- Maximum usable frequency (MUF) — the highest frequency the ionosphere will still return for that path length and time; above it the signal escapes. Longer paths use shallower angles and support a higher MUF.
- Lowest usable frequency (LUF) — set by absorption in the lower D layer; below it the signal is swallowed before it can refract.
Between the reach of the ground wave and the point where the first hop lands lies the skip zone, a ring of no reception. Signals arriving by several paths of slightly different length interfere, producing the characteristic selective fading and “flutter” of HF sky-wave reception. Because ionisation tracks the sun, the usable frequencies swing with time of day, season, and the 11-year solar cycle — low bands work at night, high bands in daytime.
In practice
Sky wave is the backbone of the HF services: international shortwave broadcast, amateur DX, marine and aeronautical long-haul, and military over-the-horizon links. Operators pick a band near the day’s MUF for the best signal-to-noise, dropping to lower bands after dark. A short, steep variant — NVIS — deliberately aims almost straight up to fill the skip zone for regional coverage. Multi-hop paths can circle the globe, but each ground reflection and each ionospheric pass adds loss and distortion.
Relevance to SDR
Hearing sky wave needs an HF-capable front end: an upconverter ahead of an RTL-SDR or a direct-sampling HF receiver such as the Airspy HF+. For a VHF/UHF trunking scanner like GopherTrunk, sky wave is out of band — land-mobile trunked systems operate well above the HF range and rely on line-of-sight coverage. The concept still matters to SDR users as the clearest illustration of why the HF spectrum behaves so unlike the bands a scanner monitors, and why the same dial can sound completely different by day and by night.