Чтение общенаучной литературы. Кытманова О.А. - 42 стр.

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43
reflected again from the Earths surface back to the ionosphere only to be reflected
once more I his bouncing process may be repeated several times, and in this way
signals can reach areas alter dark reach they cannot reach during the day.
The high frequency (HF) or short waveband is most important in long distance
communications although for many years it was regarded as useless for anything
other than short-range broadcasts. This was because the ground wav e extends only
a short distance and nobody knew anything about the sky wave until the 1920s
Then it was discovered that the sky wave penetrates the lower layers of the
ionosphere but is reflected by the Appleton layer higher up. The first reflection
returns to Earth at a point far beyond the limit of the ground wave and, by
making several ‘’bounces’’ can reach immense distances -even around the
world. Between the limit of the ground wave and the first bounce is a zone where
no signals can be heard. I his is because t what is known as the skip distance effect.
The atmospheric effects known as aurorae are caused by high-energy particles
from the Sun entering the atmosphere. These particles also interact with the Faith's
magnetic field and the ionosphere to produce sudden disturbances or radio fade
outs. The sky waves of very high frequency (VHF) lira high frequency (UHF), and
super high frequency (SHF) radio stations penetrate all the layers of ionization and
escape into space. Their ground waves become shorter in range with increasing
frequency until at the upper end of the SHF band the range does not extend much
beyond the horizon. Engineers try to increase this distance as much as possible by
siting the stations on hills or mountains.
In spite of their short ranges these bands ire very useful VHF and UHF signals
can carry a lot of information in a short time which is necessary for example for
television.
Such TV stations and TV receivers are said to have a wide_ bandwidth. The
short range of VHF and UHF stations 35-60 kilometers 20-40 miles) means that
many television stations are needed to cover even an area as small as the British
Isles!
The large amount of information that can be earned by high-frequency radio
waves makes hem useful for carrying telephone and facsimile messages. The
higher the frequency the greater the amount of information that can he carried. For
example , an SHF wave can deal with up to 900 telephone conversations at the
same time. The short-range limitation can be overcome by placing a chain of
stations at intervals between two cities and repeating the messages along the line
from one station to the next. This can be done automatically and distances of up to
1 500 kilometers (900 miles 1 can be crossed linking cities separated by deserts or
forests.
If a VHF wave is directed skyward at a low angle most of the energy escapes into
space. But a small fraction of it is scattered by the ionosphere and some of this
returns to Earth at a distant point where it can be picked up by a highly sensitive
receiver. This approach called ‘’ionospheric scatter’’ provides reliable reception
over distances of about 1 500 kilo meters (900 miles).
The scatter principle is also used for medium range communication using the
ultra-high frequencies. Here the scattering is caused by pockets of turbulence in the

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