Чтение общенаучной литературы. Кытманова О.А. - 44 стр.

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45
Today, the thermionic tube has been largely replaced by the transistor although it is
still used where large output powers are required.
How Radio Works
When a radio broadcast is being made, sound waves, vibrating at different
frequencies are picked up in the studio by microphones and changed into electrical
oscillations of the same frequencies. These signals are then passed through
amplifiers to magnify them. Higher-frequency radio waves are generated in the
transmitter. Their frequency or amplitude is then varied, or modulated, in
accordance with the audio frequency signals: they therefore "carry" the information
of the audio frequencies and are known as "carrier waves". The modulated signal is
then fed into the antenna from which they radiate in the form of electromagnetic
waves. In amplitude modulation, or AM, systems, the electrical oscillations change
the amplitude of the high-frequency carrier waves. In frequency modulation, FM
systems, the amplitude of the carrier wave remains the same but its frequency
varies with the audio-frequencies, Television transmitters use AM to transmit
video signals and FM, usually, to transmit the sound frequency modulated (FM)
wave (voice & carrier wave) How a carrier wave is modulated to broadcast speech.
Inside the radio receiver is an arrangement of inductors (coils) and capacitors
These provide the "tuning", so that the circuit accepts the correct signal and rejects
others. Transistors (or, less often, valves) amplify the signals, which are weak
when they arrive. The audio-frequencies are separated and operate a loudspeaker,
which changes the electrical oscillations back into sound vibrations.
Satellite Transmissions
By far the most important advance in recent years has been the widespread use of
satellites in Earth orbit . In "along-the-ground" broadcasting, the ranges that VHP
and other high frequency bands can achieve is roughly limited to just beyond the
horizon. Bouncing the signals off a satellite in space extends the "horizon' very
considerably.
Suppose that a UHF station in Britain wishes to contact a similar station in the
United States. Normally, this cannot be done bt cause of the curvature of the Earth.
If, however, a satellite is placed in orbit above the Atlantic there is a clear line of
sight between it and both ground stations. Signals are transmitted from the first
station to the satellite, v here a receiver picks them up and amplifies them. They
are then passed to a small transmitter in the satellite, which beams them towards
the receiving station, where they arc again amplified. One such amplifier is the
maser, a word obtained from the initials of Microwave Amplification by
Stimulated Emission of Radiation .
A communications satellite of this type contains all the necessary transmitting and
receiving equipment, usually in duplicate. It is placed in orbit by a rocket with
great precision, so that it will remain at a height of about 36,000 kilometers
(22,000 miles) over a chosen spot on the Earth's surface. This is known as a
"geostationary orbit". In this way the satellite is always in the correct position
relative to the transmitter and receiving stations on the Earth. Satellites are
powered by batteries or by solar cells (which change the Sun's rays into

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