USES OF RADIO WAVES
The prime purpose of radio is to convey
information from one place to another through the intervening
media (i.e., air, space, nonconducting materials) without
wires.
Besides being used for
transmitting sound and television signals, radio is
used for the transmission of data in coded form. In
the form of radar it is used also for sending out signals
and picking up their reflections from objects in their
path.
Long-range radio signals
enable astronauts to communicate with the earth from
the moon and carry information from space probes as
they travel to distant planets . For navigation of ships
and aircraft the radio range, radio compass (or direction
finder), and radio time signals are widely used. Radio
signals sent from global positioning satellites can
also be used by special receivers for a precise indication
of position. Various remote-control devices, including
rocket and artificial satellite operations systems and
automatic valves in pipelines, are activated by radio
signals.
The development of the
transistor and other microelectronic devices led to
the development of portable transmitters and receivers.
Cellular and cordless telephones are actually radio
transceivers. Many telephone calls routinely are relayed
by radio rather than by wires; some are sent via radio
to relay satellites.
Some celestial bodies and
interstellar gases emit relatively strong radio waves
that are observed with radio telescopes composed of
very sensitive receivers and large directional antennas.
TRANSMISSION AND RECEPTION OF RADIO WAVES
For the propagation and interception
of radio waves, a transmitter and receiver are employed.
A radio wave acts as a carrier of information-bearing
signals; the information may be encoded directly on
the wave by periodically interrupting its transmission
(as in dot-and-dash telegraphy) or impressed on it by
a process called modulation. The actual information
in a modulated signal is contained in its sidebands,
or frequencies added to the carrier wave, rather than
in the carrier wave itself. The two most common types
of modulation used in radio are amplitude modulation
(AM) and frequency modulation (FM). Frequency modulation
minimizes noise and provides greater fidelity than amplitude
modulation, which is the older method of broadcasting.
In its most common form,
radio is used for the transmission of sounds (voice
and music) and pictures (television). The sounds and
images are converted into electrical signals by a microphone
(sounds) or video camera (images), amplified, and used
to modulate a carrier wave that has been generated by
an oscillator circuit in a transmitter. The modulated
carrier is also amplified, then applied to an antenna
that converts the electrical signals to electromagnetic
waves for radiation into space. Such waves radiate at
the speed of light and are transmitted not only by line
of sight but also by deflection from the ionosphere.
Receiving antennas intercept
part of this radiation, change it back to the form of
electrical signals, and feed it to a receiver. The most
efficient and most common circuit for radio-frequency
selection and amplification used in radio receivers
is the superheterodyne. In that system, incoming signals
are mixed with a signal from a local oscillator to produce
intermediate frequencies (IF) that are equal to the
arithmetical sum and difference of the incoming and
local frequencies. One of those frequencies is applied
to an amplifier. Because the IF amplifier operates at
a single frequency, namely the intermediate frequency,
it can be built for optimum selectivity and gain. The
tuning control on a radio receiver adjusts the local
oscillator frequency. If the incoming signals are above
the threshold of sensitivity of the receiver and if
the receiver is tuned to the frequency of the signal,
it will amplify the signal and feed it to circuits that
demodulate it, i.e., separate the signal wave itself
from the carrier wave.
There
are certain differences between AM and FM receivers.
In an AM transmission the carrier wave is constant in
frequency and varies in amplitude (strength) according
to the sounds present at the microphone; in FM the carrier
is constant in amplitude and varies in frequency. Because
the noise that affects radio signals is partly, but
not completely, manifested in amplitude variations,
wideband FM receivers are inherently less sensitive
to noise. In an FM receiver, the limiter and discriminator
stages are circuits that respond solely to changes in
frequency.
The other stages of the
FM receiver are similar to those of the AM receiver
but require more care in design and assembly to make
full use of FM's advantages. FM is also used in television
sound systems. In both radio and television receivers,
once the basic signals have been separated from the
carrier wave they are fed to a loudspeaker or a display
device (usually a cathode-ray tube), where they are
converted into sound and visual images, respectively.
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