Propagation influenced by the presence of an electron gas-plasma high in the Earth's atmosphere.
The propagation of radio signals over long distances is generally restricted to the high frequency (HF) part of the spectrum (about 3 - 30 MHz), although significant propagation can take place at other frequencies in unusual circumstances.
HF signals are refracted from regions of ionisation which exist high in the Earth's atmosphere. These regions are known as the ionosphere and consist of layers of ionised gas molecules and free electrons. It is the electron gas plasma which can refract electromagnetic waves and prevent them from escaping into space. The ionised gas molecules are not thought to play a significant role at radio frequencies.
Much of the early research work into ionospheric propagation was done by Professor E.V. Appleton FRS in the 1920s at the Cavendish Laboratories in Cambridge.
The exact radio frequency that can be refracted depends on the electron density (which increases with height from the ground) and the angle of incidence of the radio waves to the ionosphere. These depend on the distance between sites, the types of aerials in use and the condition of the Ionosphere.
The amount of ionisation (and therefore the electron density in the plasma) depends on the flux of radiation (manly ultraviolet) from the Sun. These separate the electrons from the gas molecules to create the plasma and ions. The level of radiation from the Sun depends on the time of day, season of the year and the phase of an 11 year (approx.) cycle of activity. A good estimate of the level of solar activity can be found by counting the number of Sun spots that are visible on the Sun's surface. These correlate well with the solar flux level and are easy to observe with a solar telescope. DO NOT TRY TO OBSERVE THE SUN WITH THE NAKED EYE OR DIRECTLY THROUGH A TELESCOPE OR BINOCULARS.
The diagram shows (in an exaggerated way) how the height of the ionosphere varies with time of day. During the day, the solar flux increases as the Sun comes into view and the electron density in the ionosphere increases (the ionisation extends down through the atmosphere). This allows high frequencies (perhaps around 20 MHz or higher) to be propagated over long distances. Signals are refracted from the ionosphere and reflected back down to Earth in on or more "skips". Large distances can be supported in this way. At night when the solar flux is low, radio energy is absorbed less and the lower frequencies are propagated better.
The ionosphere is arranged in layers as follows:
D layer. This forms during the day at a height of about 60 - 90 kms from the ground. Since the atmosphere's density at this low altitude is quite high, a lot of absorption takes place especially at low frequencies (up to 7MHz). High frequencies are not as badly affected and they can penetrate to higher levels with relatively low attenuation. During the night, the D layer dissipates quickly and signals at the very bottom end of the spectrum (including medium wave and long wave) are no longer attenuated and can propagate over very long distances.
E layer. This layer is located at about 100 -125 kms from the ground. The E layer does not absorb signals as much as the D layer and can support some long-distance propagation. In the Summer (and sometimes in Winter), dense clouds of ionisation can form for short periods of time (they are thought to be the result of "shear winds" at high altitude). These patches of "sporadic E" can refract radio signals well into the VHF part of the spectrum. Sporadic E is responsible for long distance reception of VHF Band II signals and is often used by radio amateurs for long distance VHF operating (distances of 2000 miles or more have been achieved).
F layer. Actually, this layer can split into two: F1 and F2 when the solar flux is high (during the day). These layers tend to merge together at night into a single layer. During the day, the F1 layer forms at about 200 kms and the F2 at about 400 kms. The bulk of HF (shortwave) transmissions are propagated by the F2 layer. The altitude of the F layers is so high that it is illuminated by the Sun for long periods of the day and is slow to dissipate at night. The layer is at its minimum just before sunrise and reaches its peak just after midday.
Anomalous forms of ionospheric propagation include:
Fading
You can hear propagation bulletins on WWV (on 5.000 or 10.000 MHz) and a detailed propagation forecast at the end of the "Media Network" programme on Radio Netherlands each week. Alternatively, you can obtain reports direct from IPS Radio and Space Services by subscribing to the rec.radio.info Internet Usenet news group. They post daily reports there and you can also obtain archive data from their website. IPS Radio and Space Services can be contacted as follows:
IPS Radio and Space Services | email: office@ips.gov.au
PO Box 5606 | WWW: http://www.ips.gov.au/
West Chatswood NSW 2057 AUSTRALIA | FTP: ftp://ftp.ips.gov.au/users/rwc/
tel: +61 2 4148300 | fax: +61 2 4148331
