The ionosphere: a radio mirror powered by the Sun
About 60 to 400 km above the ground, ultraviolet and X-ray radiation from the Sun knocks electrons off atoms and creates layers of charged gas called the ionosphere. A radio wave entering this region is bent back towards Earth if the electron density is high enough for its frequency. This is called refraction, and it is what lets a 10-watt signal on 20 metres be heard on another continent.
The layers behave differently. The D layer (about 60-90 km) exists only in daylight and mainly absorbs low frequencies. The E layer (about 100-120 km) is useful for shorter hops and for sporadic-E openings. The F layer (about 200-400 km, split into F1 and F2 by day) does most of the long-distance work, because it stays ionised after sunset and reaches the highest densities.
MUF, LUF and the usable window
Every path has a Maximum Usable Frequency (MUF): above it, signals pass through the ionosphere and are lost to space. Below the Lowest Usable Frequency (LUF), signals are absorbed (mostly in the D layer) before they get through. The workable band lies between the two, and the best results are often found slightly below the MUF.
The MUF rises with solar activity and changes with the time of day, the season and the path. That is why the same band can be dead at noon and wide open at dusk. The solar flux index is the quickest single number to estimate how high the MUF can go.
Skip distance and the skip zone
A signal that returns from the ionosphere lands a long way from the transmitter, and the area in between receives nothing from the sky wave: this is the skip zone. Lower take-off angles give longer hops, and several hops (ionosphere, ground, ionosphere again) can carry a signal thousands of kilometres.
What changes propagation
Time of day: high bands favour daylight, low bands favour darkness; see HF bands by day and night. Solar cycle: the roughly 11-year cycle raises and lowers ionisation, see the sunspot number. Space weather: flares cause radio blackouts and geomagnetic storms disturb the F layer, see the K-index and A-index.
To see how all of this adds up right now, open the live HF band conditions page.
