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