|
|
| |
|
|
| |
|
|
|
|
| |
 |
| FM broadcast
transmitter of radio station KWNR, Las Vegas
which transmits on 95.5 MHz with a power of 35
kW. |
Radio
Radio is a way to send electromagnetic signals over a
long distance, to deliver information from one place to
another. A machine that sends radio signals is called a
transmitter, while a machine that "picks up" the signals
is called a receiver. A machine that does both jobs is a
"transceiver". When radio signals are sent out to many
receivers at the same time, it is called a broadcast.
Television also uses radio signals to send pictures and
sound. Radio signals can start engines moving so that
gates open on their own from a distance. (See: Radio
control.). Radio signals can be used to lock and unlock
the doors in a car from a distance.
Sound can be sent by radio, sometimes through Frequency
Modulation (FM) or Amplitude Modulation (AM).
History of radio
Many people worked to make radio possible. After James
Clerk Maxwell predicted them, Heinrich Rudolf Hertz in
Germany first showed that radio waves exist. Guglielmo
Marconi in Italy made radio into a practical tool of
telegraphy, used mainly by ships at sea. He is sometimes
said to have invented radio. Later inventors learned to
transmit voices, which led to broadcasting of news,
music and entertainment. |
|
Uses of radio
Radio was first created as a way to send telegraph
messages between two people without wires, but soon
two-way radio brought voice communication, including
walkie-talkies and eventually mobile phones.
Now an important use is to broadcast music, news and
entertainers including "talk radio". Radio shows were
used before there were TV programs. In the 1930s the US
President started sending a message about the country
every week to the American people. Companies that make
and send radio programming are called radio stations.
These are sometimes run by governments, and sometimes by
private companies, who make money by sending
advertisements. Other radio stations are supported by
local communities. These are called community radio
stations. In the early days manufacturing companies
would pay to broadcast complete stories on the radio.
These were often plays or dramas. Because companies who
made soap often paid for them, these were called "soap
operas".
Radio waves are still used to send messages between
people. Talking to someone with a radio is different
than "talk radio". Citizens band radio and amateur radio
use specific radios to talk back and forth. Policemen,
firemen and other people who help in emergency use a
radio emergency communication system to communicate
(talk to each other). It is like a mobile phone, (which
also uses radio signals) but the distance they reach is
shorter and both people must use the same kind of radio.
The word "radio" is sometimes used to mean only
voiceband broadcasting. Most voiceband broadcasting uses
lower frequency and longer wavelength than most
television broadcasting.
Microwaves have even higher frequency; shorter
wavelength. They also are used to transmit television
and radio programs, and for other purposes.
Communications satellites relay microwaves around the
world.
A radio receiver does not need to be directly in view of
the transmitter to receive programme signals. Low
frequency radio waves can bend around hills by
diffraction, although repeater stations are often used
to improve the quality of the signals.
Shortwave radio frequencies are also reflected from an
electrically charged layer of the upper atmosphere,
called the Ionosphere. The waves can bounce between the
ionosphere and the earth to reach receivers that are not
in the line of sight because of the curvature of the
Earth's surface. They can reach very far, sometimes
around the world.
Radio telescopes receive radio waves from the sky to
study astronomical objects. |
|
Applications
Below are some of the most important uses of radio,
organized by function.
Broadcasting
Broadcasting is the one-way transmission of information
from a radio transmitter to receivers belonging to a
public audience. Since the radio waves become weaker
with distance, a broadcasting station can only be
received within a limited distance of its transmitter.
Systems which broadcast from satellites can generally be
received over an entire country or continent. Older
terrestrial radio and television paid for by commercial
advertising or governments. In subscription systems like
satellite television and satellite radio the customer
pays a monthly fee. In these systems the radio signal is
encrypted and can only be decrypted by the receiver,
which is controlled by the company and can be
deactivated if the customer doesn't pay his bill.
Audio
Analog audio is the earliest form of radio broadcast. AM
broadcasting dates to the early 1900s. FM broadcasting
was introduced in the late 1930s with improved fidelity.
Time and frequency
Governments operate long-range radio standard frequency
and time signal services. These are implement by time
stations which continuously broadcast extremely accurate
time signals produced by atomic clocks, as a reference
to synchronize other clocks. Examples are BPC, DCF77,
JJY, MSF, RTZ, TDF, WWV, and YVTO. One use is in radio
clocks and watches, which include an automated receiver
which periodically (usually weekly) receives and decodes
the time signal and resets the watch's internal quartz
clock to the correct time, thus allowing a small watch
or desk clock to have the same accuracy as an atomic
clock. Government time stations are declining in number
because GPS satellites and the Internet Network Time
Protocol (NTP) provide equally accurate time standards.
Space communication
This is radio communication between a spacecraft and an
Earth-based ground station, or another spacecraft.
Communication with spacecraft involves the longest
transmission distances of any radio links, up to
billions of kilometers for interplanetary spacecraft. In
order to receive the weak signals from distant
spacecraft, satellite ground stations use large
parabolic "dish" antennas up to 25 metres (82 ft) in
diameter and extremely sensitive receivers. High
frequencies in the microwave band are used, since
microwaves pass through the ionosphere without
refraction, and at microwave frequencies the high gain
antennas needed to focus the radio energy into a narrow
beam pointed at the receiver are small and take up a
minimum of space in a satellite. Portions of the UHF, L,
C, S, ku and ka band are allocated for space
communication. A radio link which transmits data from
the Earth's surface to a spacecraft is called an uplink,
while a link which transmits data from the spacecraft to
the ground is called a downlink.
Radar
Radar is a radiolocation method used to locate and track
aircraft, spacecraft, missiles, ships, vehicles, and
also to map weather patterns and terrain. A radar set
consists of a transmitter and receiver. The transmitter
emits a narrow beam of radio waves which is swept around
the surrounding space. When the beam strikes a target
object, radio waves are reflected back to the receiver.
The direction of the beam reveals the object's location.
Since radio waves travel at a constant speed close to
the speed of light, by measuring the brief time delay
between the outgoing pulse and the received "echo", the
range to the target can be calculated. The targets are
often displayed graphically on a map display called a
radar screen. Doppler radar can measure a moving
object's velocity, by measuring the change in frequency
of the return radio waves due to the Doppler effect. |
|
Remote control
Radio remote control is the use of electronic control
signals sent by radio waves from a transmitter to
control the actions of a device at a remote location.
Remote control systems may also include telemetry
channels in the other direction, used to transmit
real-time information of the state of the device back to
the control station. Unmanned spacecraft are an example
of remote controlled machines, controlled by commands
transmitted by satellite ground stations. Most handheld
remote controls used to control consumer electronics
products like televisions or DVD players actually
operate by infrared light rather than radio waves, so
are not examples of radio remote control. A security
concern with remote control systems is spoofing, in
which an unauthorized person transmits an imitation of
the control signal to take control of the device. |
|
 Kiddle: Radio
Wikipedia: Radio |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Search Fun Easy English |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
About
Contact
Copyright
Resources
Site Map |
