Encyclopedia
Radio is the wireless transmission of signals, by modulation of
electromagnetic waves with
frequencies below those of
light.
Radio waves
Radio waves are a form of electromagnetic radiation, created whenever a charged object
accelerates with a frequency that lies in the
radio frequency portion of the
electromagnetic spectrum. In radio, this acceleration is caused by an
alternating current in an antenna. Radio frequencies occupy the range from a few tens of hertz to three hundred gigahertz, although commercially important uses of radio use only a small part of this spectrum.
ELF - SLF - ULF/VF - VLF - LF/LW - MW - HF/SW - VHF - UHF - SHF - EHF
Electromagnetic radio spectrum |
Other types of electromagnetic radiation, with frequencies above the RF range, are
microwave,
infrared, visible
light,
ultraviolet,
X-rays and
gamma rays. Since the energy of an individual
photon of radio frequency is too low to remove an
electron from an
atom, radio waves are classified as non-ionizing radiation.
Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. It does not require a medium of transport . When radio waves pass an electrical conductor, the oscillating electric or magnetic field induces an alternating current and voltage in the conductor. This can be transformed into audio or other signals that carry information. The word 'radio' is used to describe this phenomenon, and
television, radio, and cell phone transmissions are all classed as radio frequency emissions.
History and invention
Originally, radio technology was called 'wireless telegraphy', which was shortened to 'wireless'. The prefix
radio- in the sense of wireless transmission was first recorded in the word
radioconductor, coined by the French physicist Edouard Branly in 1897 and based on the verb
to radiate. 'Radio' as a noun is said to have been coined by advertising expert Waldo Warren . The word appears in a 1907 article by
Lee de Forest, was adopted by the
United States Navy in 1912 and became common by the time of the first commercial broadcasts in the United States in the 1920s. The American term was then adopted by other languages in Europe and Asia, although Britain retained the term 'wireless' until the mid-
20th century. In Chinese, the term 'wireless' is the basis for the term 'radio wave' although the term for the device that listens to radio waves is literally 'device for receiving sounds'.
Invention
The identity of the original
inventor of radio, at the time called wireless telegraphy, is contentious. The controversy over who invented the radio, with the benefit of hindsight, can be broken down as follows:
- Guglielmo Marconi was an early radio experimenter and founded the first commercial organization devoted to the development and use of radio.
- Nikola Tesla developed means to reliably produce radio frequencies, publicly demonstrated the principles of radio, and transmitted long-distance signals. He holds the US patent for the invention of the radio, as defined as "wireless transmission of data."
- Alexander Stepanovich Popov, in 1894, built his first radio receiver, which contained a coherer. Further refined as a lightning detector, he presented it to the Russian Physical and Chemical Society on May 7, 1895.
- Reginald Fessenden and Lee de Forest invented amplitude-modulated radio, so that more than one station can send signals .
- Edwin H. Armstrong invented frequency-modulated radio, so that an audio signal can avoid "static," that is, interference from electrical equipment and atmospherics.
Early radios ran the entire power of the transmitter through a carbon microphone. While some early radios used some type of amplification through electric current or battery, until the mid
1920s the most common type of receiver was the
crystal set. In the 1920s, amplifying
vacuum tube radio receivers and
transmitters came into use.
Brief history
In 1893 in
St. Louis,
Missouri, Tesla made devices for his experiments with
electricity. Addressing the
Franklin Institute in
Philadelphia and the
National Electric Light Association, he described and demonstrated in detail the principles of his work. The descriptions contained all the elements that were later incorporated into radio systems before the development of the
vacuum tube. He initially experimented with magnetic receivers, unlike the coherers used by
Guglielmo Marconi and other early experimenters. . Tesla is usually considered the first to apply the mechanism of electrical conduction to wireless practices.
In 1896 Marconi was awarded the
British patent 12039,
Improvements in transmitting electrical impulses and signals and in apparatus there-for, for radio. In 1897 he established the world's first radio station on the
Isle of Wight,
England. Marconi opened the world's first "wireless" factory in Hall Street,
Chelmsford, England in 1898, employing around 50 people. Around 1900, Tesla opened the
Wardenclyffe Tower facility and advertised services. By 1903, the tower structure neared completion. Various theories exist on how Tesla intended to achieve the goals of this wireless system . Tesla claimed that Wardenclyffe, as part of a world system of transmitters, would have allowed secure multichannel transceiving of information, universal navigation, time synchronization, and a global location system.
The next great invention was the
vacuum tube detector, invented by a team of
Westinghouse engineers. On
Christmas Eve, 1906,
Reginald Fessenden used a synchronous rotary-spark transmitter for the first radio program broadcast, from Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing
O Holy Night on the
violin and reading a passage from the
Bible. The first radio news program was broadcast August 31, 1920 by station 8MK in
Detroit, Michigan. The first college radio station, 2ADD, renamed WRUC in 1940, began broadcasting October 14, 1920 from
Union College,
Schenectady,
New York. The first regular entertainment broadcasts commenced in 1922 from the
Marconi Research Centre at
Writtle, near
Chelmsford, England.
One of the first developments in the early
20th century was that
aircraft used commercial AM radio stations for navigation. This continued until 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. Radio was used to transmit pictures visible as
television as early as the
1920s. Standard analog transmissions started in North America and Europe in the 1940s. In 1954, Regency introduced a pocket
transistor radio, the
TR-1, powered by a "standard 22.5 V Battery".
In 1960,
Sony introduced its first transistorized radio, small enough to fit in a vest pocket, and able to be powered by a small battery. It was durable, because there were no tubes to burn out. Over the next 20 years, transistors replaced tubes almost completely except for very high-power uses. In 1963 color television was commercially transmitted, and the first
communication satellite, TELSTAR, 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. Digital transmissions began to be applied to broadcasting in the late 1990s.
Uses of radio
Many of radio's early uses were maritime, for sending telegraphic messages using
Morse code between ships and land. The earliest users included the Japanese Navy scouting the Russian fleet during the
Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the
RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors.
Radio was used to pass on orders and communications between armies and navies on both sides in
World War I; Germany used radio communications for diplomatic messages once its submarine cables were cut by the British. The United States passed on President
Woodrow Wilson's Fourteen Points to Germany via radio during the war.
Broadcasting began to become feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s.
Another use of radio in the pre-war years was the development of detecting and locating aircraft and ships by the use of
radar .
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
AM broadcast radio sends music and voice in the Medium Frequency radio spectrum. AM radio uses
amplitude modulation, in which louder sounds at the microphone causes wider fluctuations in the transmitter power while the transmitter frequency remains unchanged. Transmissions are affected by static because lightning and other sources of radio add their radio waves to the ones from the transmitter.
FM broadcast radio sends music and voice, with higher fidelity than AM radio. In
frequency modulation, louder sounds at the microphone cause the transmitter frequency to fluctuate farther, the transmitter power stays constant. FM is transmitted in the Very High Frequency radio spectrum. VHF radio waves act more like light, travelling in straight lines, hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the
ionosphere, resulting in
Long distance FM reception. 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 receivers are relatively immune to lightning and spark interference.
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, background music or stereo sound signals. 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.
Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. . Aircraft fly high enough that their transmitters can be received hundreds of miles away, even though they are using VHF.
Marine voice radios can use AM in the shortwave High Frequency radio spectrum for very long ranges or
narrowband FM in the VHF spectrum for much shorter ranges. 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 kHz of deviation, rather than the 75 kHz used by FM broadcasts and 25 kHz used by TV sound.
Civil and military HF voice services use
shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice , which uses less bandwidth than AM. On an AM radio SSB sounds like ducks quacking. 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 suppressing the carrier and 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,
Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances. Commercial services such as
XM,
WorldSpace and
Sirius offer encrypted digital Satellite radio.
Telephony
Cell phones transmit to a local
cell site that ultimately connects to the public switched telephone network through an optic fiber or microwave radio and other network elements. When the mobile phone nears the edge of the cell site's radio coverage area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital modulation schemes. 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 uses 66 Low Earth Orbit satellites as the cells.
Video
Television sends the picture as AM and the sound as FM, with the sound carrier a fixed frequency away from the video carrier. Analog televison also uses a vestigial sideband on the video carrier to reduce the bandwidth required.
Digital television uses
quadrature amplitude modulation. A
Reed-Solomon error correction code adds redundant correction codes and allows reliable reception during moderate data loss. Although many current and future codecs can be sent in the MPEG-2
transport stream container format, as of 2006 most systems use a standard-definition format almost identical to
DVD: MPEG-2 video in
Anamorphic widescreen and MPEG layer 2 audio.
High-definition television is possible simply by using a higher-resolution picture, but H.264/AVC is being considered as a replacement video codec in some regions for its improved compression. With the compression and improved modulation involved, a single "channel" can contain a high-definition program and several standard-definition programs.
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.
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.
Loran systems also used time-of-flight radio signals, but from radio stations on the ground.
VOR systems , have an antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing or radial from the station, thus establishing a line of position. An aircraft can get readings from two VORs, and locate its position at the intersection of the two radials, known as a "fix." When the VOR station is collocated with DME , the aircraft can determine its bearing and range from the station, thus providing a fix from only one ground station. Such stations are called VOR/DMEs. The military operates a similar system of navaids, called TACANs, which are often built into VOR stations. Such stations are called VORTACs. Because TACANs include distance measuring equipment, VOR/DME and VORTAC stations are identical in navigation potential to civil aircraft.
Radar
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 Radio Beacons , 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
Most new radio systems are digital, see also:Digital TV, Satellite Radio, Digital Audio Broadcasting. The oldest form of digital broadcast was spark gap
telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in
Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful of both radio frequencies and power.
The next advance was continuous wave
telegraphy, or CW , in which a pure radio frequency, produced by a
vacuum tube electronic oscillator was switched on and off by a key. A receiver with a local oscillator would "heterodyne" with the pure radio frequency, creating a whistle-like audio tone. CW uses less than 100 Hz of bandwidth. CW is still used, these days primarily by
amateur radio operators . Strictly, on-off keying of a carrier should be known as "Interrupted Continuous Wave" or ICW.
Radio teletypes usually operate on short-wave 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. From about 1925 to 1975, radio teletype was how most commercial messages were sent to less developed countries. These are still 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. Microwave dishes on satellites, telephone exchanges and TV stations usually use
quadrature amplitude modulation . 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.
Systems that need reliability, or that share their frequency with other services, may use "corrected orthogonal frequency-division multiplexing" or
COFDM. COFDM breaks a digital signal into as many as several hundred slower subchannels. The digital signal is often sent as QAM on the subchannels. Modern COFDM systems use a small computer to make and decode the signal with digital signal processing, which is more flexible and far less expensive than older systems that implemented separate electronic channels. COFDM resists fading and ghosting because the narrow-channel QAM signals can be sent slowly. An adaptive system, or one that sends error-correction codes can also resist interference, because most interference can affect only a few of the QAM channels. COFDM is used for WiFi, some
cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards.
Heating
Radio-frequency energy generated for heating of objects is generally not intended to radiate outside of the generating equipment, to prevent interference with other radio signals.
Microwave ovens use intense radio waves to heat food. Diathermy equipment is used in surgery for sealing of blood vessels. Induction
furnaces are used for melting metal for
casting.
Mechanical force
Tractor beams can use radio waves which exert small electrostatic and magnetic forces. These are enough to perform station-keeping in microgravity environments. Conceptually,
spacecraft propulsion: Radiation pressure from intense radio waves has been proposed as a propulsion method for an interstellar probe called Starwisp. Since the waves are long, the probe could be a very light metal mesh, and thus achieve higher accelerations than if it were a
solar sail.
Other
Amateur radio is a hobby in which enthusiasts purchase or build their own equipment and use radio for their own enjoyment. They may also provide an emergency and public-service radio service. This has been of great use, saving lives in many instances. Radio amateurs are able to use frequencies in a large number of narrow bands throughout the radio spectrum. They 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.
Personal radio services such as
Citizens' Band Radio, Family Radio Service, Multi-Use Radio Service and others exist in North America to provide simple, short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world. A number of schemes have been proposed Wireless energy transfer. Various plans included transmitting power using
microwaves, and the technique has been demonstrated. . These schemes include, for example,
solar power stations in orbit beaming energy down to terrestrial users.
Radio remote control use sof radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats,
cars and aeroplanes. Large industrial remote-controlled equipment such as cranes and switching
locomotives now usually use digital radio techniques to ensure safety and reliability. Energy autarkic radio technology consists of a small radio transmitter powered by environmental energy .
See also
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