What changed
Propagation — edited Aug 31, 2026 by Unknown editor
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Radio waves above about 3 MHz would normally escape into space. The **ionosphere** — layers of
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atmosphere ionized by solar radiation — bends them back down, which is what makes worldwide
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shortwave communication possible.
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## The layers
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- **D layer** (60–90 km) — exists only in daylight, and *absorbs* low-frequency signals. This is why
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80 m and 40 m are short-range by day and open up at sunset.
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- **E layer** (100–125 km) — mostly daytime. **Sporadic E** is intense, patchy ionization that opens
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10 m, 6 m and occasionally 2 m for continental distances, mainly in early summer.
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- **F layer** (200–400 km, splitting into F1/F2 by day) — the workhorse for HF DX.
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## The solar cycle
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Sunspot numbers rise and fall on an eleven-year cycle. At solar maximum the higher bands (10, 12,
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15, 17 m) open worldwide daily; at minimum they are often dead and activity concentrates on 40 and
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20 m. Numbers to watch: **solar flux index (SFI)** — higher is better; **A and K indices** — lower
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is better, and a K of 5 or more means geomagnetic disturbance and degraded paths, especially
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northern ones.
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## Practical patterns
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- **Grey line** — the terminator at dawn and dusk gives a brief window of exceptional low-band
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propagation.
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- **MUF** — the maximum usable frequency for a path right now; the best band is usually just below
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it.
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- **NVIS** — a low horizontal antenna sends signals nearly straight up for reliable coverage out to
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300 miles, filling the "skip zone". Standard practice for regional emergency nets.
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## Tools
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Real-time [FT8](/wiki/operating/digital-modes/ft8) reception maps on PSK Reporter and the Reverse
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Beacon Network tell you what is open right now, far better than any prediction. VOACAP predicts
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paths in advance; propagation beacons on 14.100 MHz cycle around the world continuously.