Чтение общенаучной литературы. Кытманова О.А. - 40 стр.

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41
range of the new wireless apparatus was only a few kilometers and sending
messages was slow work There was no way of tuning, and so two transmitters
within range of each other could not work at the same time If they did, the
receivers picked up both messages simultaneously, with chaotic results.
But between 1896 and 1901 Marconi made great progress. Transmitters grew
from a simple, battery-operated, table-top version to a high-power station on the
cliffs at Poldhu in the English county of Cornwall The tuned circuit was
developed, so that stations could operate without interfering with one another, and
receivers were improved. In December 1901 Marconi announced that signals from
Poldhu had crossed the Atlantic Ocean and been received at St John's in
Newfoundland.
The Ionosphere
Yet, in the light of the knowledge of the time this was theoretically impossible.
Hert had shown that radio waves traveled in optical paths It was thought that to
cross the Atlantic antennae over 150 kilometers (93 miles t high would he needed
on each side because of the curvature of the Earth. Marconi had no independent
witnesses and his claim was not believed at first. What was causing the waves to
travel like this when in theory the> should have left the Earth's surface just beyond
the horizon to vanish into space? No one knew. But in 1902 an American electrical
engineer, Arthur E. Kennelly, and an English physicist, Oliver Heaviside,
suggested that high above the Earth was a layer ionization (electrified particles)
which acted as a mirror, reflecting the radio waves down to Earth again. Few
people accepted this ,it first However in l924 the Kennelly-Heaviside layer was
detected experimentally by the British scientist Edward Appleton.
In fact several regions (D, E, F1, and F2) of ionization surround the Earth, at
different levels and in two main groups. They are caused by the Sun's action on
gases in the upper atmosphere, where ultraviolet radiation frees some of the
electrons from the gas atoms. These free electrons reflect part of the radio wave
back to Earth while the remainder is absorbed by the ionosphere. The amount
reflected depends on various factors, including the density of the layer, its height,
the frequency of the radio wave and the angle at which it encounters the layer.
One of the two main layers (from 80 to 140 kilometers, or 50 to 90 miles, above
the Earth) is called the Kennelly—Heaviside layer (region E). The other, known as
the Appleton layer (regions FI and F>). is about 240 kilometers 1150 miles) up.
These layers made longdistance radio transmissions possible before the first
satellite launches.
The Radio Wave Spectrum
The spectrum of radio waves is divided into seven main bands. The divisions are
artificial ones, made on the basis of different transmission characteristics. In each
case, there is a gradual shift from one band to the next, rather than a sudden break.
The bands are described in terms of wavelength or frequency. These values are
directly related: wavelength is the velocity of electromagnetic radiation (300,000
kilometers or 186,000 miles per second) divided by frequency. A high frequency
corresponds to a short wavelength and vice versa
In ascending order, the first division is the very low frequency (VLF) band Only a

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