Чтение общенаучной литературы. Кытманова О.А. - 13 стр.

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14
By talking or singing into the microphone the diaphragm inside moves back and
forth with the sound vibrations, thus altering the steady current previously flowing
around the circuit DMX. Current pulsations in X, the transformer primary, cause
similar pulsations in Z, the secondary . circuit carrying the plate current. The effect
of the relatively low frequency audio currents on the high frequency oscillations
already there is to alter their amplitude.
The modulated oscillations are induced in the antenna circuit by resonance, and
are radiated as electromagnetic waves of the same frequency and form. The
continuous wave produced by the radio frequency oscillations alone is called the
carrier wave, the alteration of its amplitude by audio frequencies being spoken of
as modulation. Although radio transmitters with one vacuum tube have been used
by radio amateurs, it is customary to find transmitters with half a dozen or more
tubes. The principal function of additional tubes in receivers as well as transmitters
is to amplify currents wherever they are needed and thereby give greater
transmitting range and clearer reception.
Radio Receivers
Transmission of intelligence by radio is based on modulation, this being a process
by which the message to be transmitted is superimposed at the sending end of a
radio link as a modulating signal on a strong carrier wave, thereby changing the
latter's amplitude, frequency or phase. The modulated carrier is radiated by a
transmitting aerial as a wave of electromagnetic energy which propagates through
space at the velocity of light. At the point of reception the modulated wave is
picked up by a receiving aerial and is fed to the receiver input. In the receiver the
signal is separated from the radio-frequency carrier and drives the receiver load,
which may be a speaker, a recorder, a cathode-ray tube, etc. As an electromagnetic
wave travels away from the transmitter it is weakened or attenuated. This is why
radio receivers should be capable of picking up relatively weak signals. Radio
serves a variety of purposes such as communication, broadcasting, navigation,
radar and telecontrol.
Radio communication is the transmission and reception of messages without
wires or wave guides. It includes communication by radio telegraph, radio-
telephone, radio teletypewriter, radio facsimile and television. It is the only method
of communication between stationary and mobile objects (e. g. from ship to shore,
from ground to aircraft, from ground to spaceships, etc.). Radio broadcasting is
radio transmission for general reception, including speech, music and commercial
television.
Radio navigation is the use of radio facilities for determining the position and
direction of ships and planes. Radar (which is an acronym for Radio Detection and
Ranging) is a technique for determining the range and bearings of objects (targets)
by transmitting beamed high-power signals against reflective targets, the reception
of the reflected signals and the presentation of the resultant data on a dial or a
cathode ray display. Telecontrol is a technique for control of machinery by radio.
There exist two classes of receivers: communication and broad-cast receivers, the

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