We use high-frequency (HF), or short-wave, radio to communicate over long distances because it is convenient, although distorted at times. Transmitters can send signals all the way round the globe if they need to by bouncing them off the ionosphere, the portion of atmosphere between 50 and 1000 kilometres above the Earth that is ionised by radiation and particles from the Sun. HF signals need 10 bounces and less than one-seventh of a second to circle the world once.
Unlike satellite communications, or SATCOMS，standard HF transmitters and receivers can be cheap, light and compact, and require little power to operate. Small HF sets are also more versatile than miniature SATCOMS. They can transmit speech, data and now still pictures, and they are suitable for people on the move: for the operators of trains, boats and planes,and for mineral prospectors in remote locations. The armed forces are particularly keen on HF radio because the ionosphere is more resilient to attack than a satellite.
HF radio operates in the waveband from 3 to 30 megahertz: the upper limit is about the maximum frequency that is influenced by the ionosphere. This mixture of ions and electrons,or plasma ,has its most significant effect on HF radio waves between altitudes of about 80 kilometres and 300 kilometres, where the concentration of free electrons is greatest. Radio scientists label regions or layers,of the ionosphere with letters. The D Region,at around 80 kilometres, attenuates HF signals that pass through it. The E Region,at around 110 kilometres,is more helpful; during the day it may be able to reflect HF transmissions. The F Region, which is subdivided into the F1 and F2 Regions between 200 and 300 kilometres, reflects waves that manage to pass through the lower layers. The plasma fizzles out above 300 kilometres.
The ionosphere tends to trap waves with frequencies less than 30 megahertz. Besides the HF band,there are waves in the medium frequency (MF)，or medium wave (MW),band from 300 to 3000 kilohertz; in the low frequency (LF)，or long wave (LW)，band from 30 to 300 kilohertz; and in the very low frequency (VLF) band from 3 to 30 kilohertz;and in the extremely low frequency (ELF) band from 300 to 3000 hertz. Waves in the ELF band can penetrate water, which makes them ideal carriers of messages to submarines. The trouble is that low frequency waves cannot carry very much data because you cannot impress fine information on a long wavelength: an ELF wave can transmit only a few bits of information per second. (The higher the frequency of a wave, the shorter the wavelength is. You can divide the speed of light, 300 000 kilometres per second, by the frequency of a wave to determine the same band position as a wavelength.
Waves with frequencies greater than 30 megahertz tend to pass through the ionosphere. These are waves in the very high frequency (VHF) band from 30 to 300 megahertz; in the ultra high frequency (UHF) band which is used for transmitting TV signals, from 300 to 3000 megahertz ；and in the super high frequency (SHF)，or microwave, band from 3000 to 30000 megahertz.
The structure of the ionosphere is variable and this means that the frequency limits for HF radio communications fluctuate. Its composition changes with the time of day, the season of the year and the sunspot cycle. The distance between terminals and the type of equipment being used also influence the limits. If radio scientists are to exploit the ionosphere fully for HF communications they need to be able to alter easily the frequency at which their equipment operates. Civil and military communicators use forecasts of the state of the global ionosphere to help them to select the best frequency to use at a particular time. For instance, the D and E Regions are hardly present at all after sunset and so waves absorbed during the day are reflected at night. However, it is the frequent changes in the composition of the F Region that present the greatest challenge；these demand skilled radio operators or the most up-to-date equipment, which emits signals to probe the ionosphere to determine its state.
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