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Radio
From Wikipedia, the free encyclopedia.
Radio is a technology that allows for the transmission of
signals by
modulation of
electromagnetic waves. These waves travel (propagate) through the air and
the vacuum of space equally well, not requiring a medium of transport.
A radio wave is created whenever a
charged object
accelerates with a frequency that lies in the
radio frequency (RF) portion of the
electromagnetic spectrum. By contrast, other types of emissions which fall
outside the RF range are
gamma rays,
X-rays,
infrared &
ultraviolet light, and light visible to humans.
When a radio wave passes a wire, it induces a moving electric charge
(voltage) that can be transformed into audio or other signals that carry
information. Although the word 'radio' is used to describe this phenomenon, the
transmissions which we know as
television,
radio,
radar, and cell phone are all in the class of radio frequency emissions.
The theoretical basis of the propagation of electromagnetic waves was first
described in
1873
by
James Clerk Maxwell in his paper to the
Royal Society A dynamical theory of the electromagnetic field,
which followed his work between
1861
and
1865.
It was
Heinrich Rudolf Hertz who, between
1886
and
1888, first validated Maxwell's theory through experiment, demonstrating
that radio radiation had all the properties of waves (then called Hertzian
waves), and discovering that the electromagnetic equations could be reformulated
into a partial differential equation called the
wave equation.
The identity of the original inventor of radio, at the time called
wireless telegraphy, is contentious.
In
1893 in
St. Louis,
Missouri,
Nikola Tesla made the first public demonstration of radio communication.
Addressing the Franklin Institute in Philadelphia and the National
Electric Light Association, he described and demonstrated in detail the
principles of radio communication. The apparatus that he used contained all the
elements that were incorporated into radio systems before the development of the
vacuum tube.
In
1894 British physicist Sir
Oliver Lodge demonstrated the possibility of signalling using radio waves
using a detecting device called a coherer, a tube filled with
iron
filings which had been invented by
Temistocle Calzecchi-Onesti at Fermo in Italy in 1884.
Edouard Branly of France and
Alexander Popov of Russia later produced improved versions of the coherer.
Popov, who developed a practical communication system based on the coherer, is
often considered by his own countrymen to have been the inventor of radio.
In
1896
Guglielmo Marconi was awarded what is sometimes recognised as the world's
first patent for radio with
British
Patent 12039, Improvements in transmitting electrical impulses and
signals and in apparatus there-for.
In
1897 in the USA, some key developments in radio's early history were created
and patented by Nikola Tesla. However the US Patent Office reversed its decision
in
1904, awarding Guglielmo Marconi a patent for the invention of radio,
possibly influenced by Marconi's financial backers in the States, who included
Thomas Edison and
Andrew Carnegie. In
1909
Marconi, with
Karl Ferdinand Braun, was also awarded the
Nobel Prize in Physics for "contributions to the development of wireless
telegraphy".
However,
Tesla's patent (number 645576) was reinstated in
1943
by the US Supreme Court, shortly after his death. This decision was based on the
fact that there was prior work existing before the establishment of Marconi's
patent. Some believe it was apparently made for financal reasons, to allow the
US Government to avoid having to the pay damages that were being claimed by the
Marconi Company for use of its patents during
World War I.
Claims have also been made that
Nathan Stubblefield invented radio before either Tesla or Marconi, but his
device seems to have worked by
induction transmission rather than radio transmission. Marconi opened the
world's first "wireless" factory in Hall Street,
Chelmsford, England in
1898,
employing around 50 people. The next great invention was the
vacuum tube detector, invented by a team of Westinghouse engineers.
On
Christmas Eve,
1906,
using his
heterodyne principle,
Reginald Fessenden transmitted the first radio audio broadcast in history
from
Brant Rock Station, Massachusetts. Ships at sea heard a broadcast that
included Fessenden playing the song O Holy Night on the violin and
reading a passage from the
Bible. The world's first regular wireless broadcasts for entertainment
commenced in
1922
from the
Marconi Research Centre at
Writtle near
Chelmsford, England, which was also the location of the world's first
"wireless" factory.
Early radios ran the entire power of the transmitter through a carbon
microphone. In the
1920s, amplifying
vacuum tubes revolutionized both
radio receivers and
radio transmitters.
Developments in the
20th century:
As a matter of course, aircraft used commercial AM radio stations for
navigation. This continued through the early
1960s when VOR systems finally became widespread.
In the early
1930s,
single sideband and
frequency modulation were invented by amateur radio operators. By the end
of the decade, they were established commercial modes.
In 1960,
Sony introduced the first
transistorized radio, small enough to fit in a vest pocket, and able to be
powered by a small battery. It was reliable, because there were no tubes to
burn out. Over the next twenty years, transistors displaced tubes almost
completely except for very high power, or very high frequency.
In 1963 color television was commercially transmitted, and the first
(radio)
communication satellite was launched.
In the late 1960s, the U.S. long-distance telephone network began to
convert to a digital network, employing
digital radios for many of its links.
In the 1970s,
LORAN became the premier radio navigation system. Soon, the U.S. Navy
experimented with
satellite navigation, culminating in the invention and launch of the
GPS
constellation in 1987.
In the early 1990s,
amateur radio experimenters began to use personal computers with audio
cards to process radio signals. In 1994, the U.S. Army and
DARPA launched an aggressive, successful project to construct a
software radio that could become a different radio on the fly by changing
software.
Many of its early uses were naval, for sending
Morse code messages between ships and land. Today, radio takes many forms,
including
wireless networks,
mobile communications of all types, as well as radio
broadcasting. Read more about radio's
history.
Before the advent of
television, commercial radio broadcasts included not only news and music,
but dramas, comedies, variety shows, and many other forms of entertainment.
Radio was unique among dramatic presentation that it used only sound. For more,
see
radio programming.
There are a number of uses of radio:
Audio
The oldest form of audio broadcast was marine radio telegraphy. A
continuous wave, or CW, was switched on and off by a key to create
Morse code, which was heard at the receiver as an intermittent tone. CW
is still used, these days primarily by amateur radio operators (hams).
AM radio sends music and voice. AM radio uses
amplitude modulation, in which higher air-pressure at the microphone
causes higher transmitter power. Transmissions are affected by static
because lightning and other sources of radio add their radio waves to the
ones from the transmitter.
FM radio sends music and voice, with higher fidelity than AM radio. In
frequency modulation, a higher air-pressure at the microphone turns into
a higher transmitted frequency. FM is transmitted as Very High Frequency
radio waves (VHF -- 30MHz to 300MHz). There are more frequencies available
at higher frequencies, so there can be more stations, each sending more
information. Another effect is that the shorter radio waves act more like
light, travelling in straight lines that are not reflected back towards the
Earth by the
ionosphere, resulting in a shorter effective reception range. FM
receivers are subject to the
capture effect, which causes the radio to only receive the strongest
signal when multiple signals appear on the same frequency.
FM Subcarrier services are secondary signals transmitted "piggyback"
along with the main program. Special receivers are required to utilize these
services. Analog channels may contain alternative programming, such as
reading services for the blind, or background music. In some extremely
crowded metropolitan areas, the subchannel program might be an alternate
foreign language radio program for various ethnic groups. Subcarriers can
also transmit digital data, such as station identification, the current
song's name, web addresses, or stock quotes. In some countries, FM radios
automatically retune themselves to the same channel in a different district
by using sub-bands.
Marine and aviation voice radios use VHF AM. AM is used so that multiple
stations on the same channel can be received. (Use of FM would result in
stronger stations blocking out reception of weaker stations due to FM's
capture effect). Aircraft are often so high that their radios can see
hundreds of miles, even though they are using VHF.
Government, police, fire and commercial voice services use narrowband FM
on special frequencies. Fidelity is sacrificed to use a smaller range of
radio frequencies, usually five kilohertz of deviation (5 thousand cycles
per second) for maximum pressure, rather than the 16 used by FM broadcasts
and TV sound.
Civil and military HF (high frequency) voice services use
shortwave radio to contact ships at sea, aircraft and isolated
settlements. Most use
single sideband voice (SSB), which uses less bandwidth than AM. SSB
sounds like ducks quacking on an AM radio. Viewed as a graph of frequency
versus power, an AM signal shows power where the frequencies of the voice
add and subtract with the main radio frequency. SSB cuts the bandwidth in
half by sacrificing the carrier and (usually) lower sideband. This also
makes the transmitter about three times more powerful, because it doesn't
need to transmit the unused carrier and sideband.
TETRA,
Terrestial Trunked Radio is a digital cell phone system for military,
police and ambulances.
Telephony
Cell phones transmit to a local cell radio, which connects to the public
service telephone network through an optic fiber or microwave radio. When
the phone leaves the cell radio's area, the central computer switches the
phone to a new cell. Cell phones originally used FM, but now most use
various digital encodings.
Satellite phones come in two types:
INMARSAT and
Iridium. Both types provide world-wide coverage. INMARSAT uses
geosynchronous satellites, with aimed high-gain antennas on the vehicles.
Iridium provides cell phones, except the cells are satellites in orbit.
Video
Television sends the picture as AM, and the sound as FM, on the same
radio signal.
Digital television encodes three bits as eight strengths of AM signal.
The bits are sent out-of-order to reduce the effect of bursts of radio
noise. A
Reed-Solomon error correction code lets the receiver detect and correct
errors in the data. Although any data could be sent, the standard is to use
MPEG-2 for video, and five CD-quality (44.1 kilo-sample/sec) digital
channels (center, left, right, left-back and right back). With all this, it
takes only half the bandwidth of an analog TV signal because the video data
is compressed.
Telecommunications
Navigation
All
satellite navigation systems use satellites with precision clocks. The
satellite transmits its position, and the time of the transmission. The
receiver listens to four satellites, and can figure its position as being on
a line that is tangent to a spherical shell around each satellite,
determined by the time-of-flight of the radio signals from the satellite. A
computer in the receiver does the math.
Loran systems also used time-of-flight radio signals, but from radio
stations on the ground.
VOR
systems (used by aircraft), have two transmitters. A directional transmitter
scans like a lighthouse at a fixed rate. When the directional transmitter is
facing north, an omnidirectional transmitter pulses. An aircraft can get
readings from two VORs, and locate its position at the intersection of the
two beams.
Radio direction-finding is the oldest form of radio navigation. Before
1960 navigators used movable loop antennas to locate commercial AM stations
near cities. In some cases they used marine radiolocation beacons, which
share a range of frequencies just above AM radio with amateur radio
operators.
Radar detects things at a distance by bouncing radio waves off them. The
delay caused by the echo measures the distance. The direction of the beam
determines the direction of the reflection. The polarization and frequency
of the return can sense the type of surface.
Navigational radars scan a wide area two to four times per minute. They
use very short waves that reflect from earth and stone. They are common on
commercial ships and long-distance commercial aircraft.
General purpose radars generally use navigational radar frequencies, but
modulate and polarize the pulse so the receiver can determine the type of
surface of the reflector. The best general-purpose radars distinguish the
rain of heavy storms, as well as land and vehicles. Some can superimpose
sonar data and map data from
GPS
position.
Search radars scan a wide area with pulses of short radio waves. They
usually scan the area two to four times a minute. Sometimes search radars
use the
doppler effect to separate moving vehicles from clutter.
Targeting radars use the same principle as search radar but scan a much
smaller area far more often, usually several times a second or more.
Weather radars resemble search radars, but use radio waves with circular
polarization and a wavelength to reflect from water droplets. Some weather
radar use the
doppler to measure wind speeds.
Emergency services
emergency position-indicating rescue beacons (EPIRBs),
emergency locating transmitters or
personal locator beacons are small radio transmitters that satellites
can use to locate a person or vehicle needing rescue. Their purpose is to
help rescue people in the first day, when survival is most likely. There are
several types, with widely-varying performance.
Data (digital
radio)
Microwave dishes on satellites, telephone exchanges and TV stations
usually use
quadrature amplitude modulation (QAM). QAM sends data by changing both
the phase and the amplitude of the radio signal. Engineers like QAM because
it packs the most bits into a radio signal. Usually the bits are sent in
"frames" that repeat. A special bit pattern is used to locate the beginning
of a frame.
IEEE 802.11, the radio network standard, has stations with
digital tuners. They start off by contacting a central control node,
which tells the nodes about each other so they can communicate privately.
Nodes move through many frequencies. They use a pseudo-random number
generator to select the next frequency.
Radio teletypes usually operate on short-wave (HF) and are much loved by
the military because they create written information without a skilled
operator. They send a bit as one of two tones. Groups of five or seven bits
become a character printed by a teletype. These are classically used by the
military and weather services.
Aircraft use a 1200 Baud radioteletype service over VHF to send their
ID, altitude and position, and get gate and connecting-flight data.
Heating
Microwave ovens use intense radio waves to heat food. (Note: It is a
common misconception that the radio waves are tuned to the resonant
frequency of water molecules. The microwave frequencies used are actually
about a factor of 10 below the resonant frequency.)
Mechanical Force
Tractor beams: Radio waves exert small electrostatic and magnetic
forces. These are enough to perform station-keeping in microgravity
environments.
Space drive: Radiation pressure from intense radio waves has been
proposed as a propulsion method for
interstellar probes. Since the waves are long, the probe could be a very
light-weight metal mesh, and thus achieve higher accelerations than if it
were a light sail.
Other
Amateur radio is an emergency and public-service radio service provided
by enthusiasts who purchase or build their own equipment. It operates in a
large number of narrow bands throughout the radio spectrum. Radio amateurs
use all forms of encoding, including obsolete and experimental ones. Several
forms of radio were pioneered by radio amateurs and later became
commercially important, including FM, single-sideband AM, digital packet
radio and satellite repeaters.
