   #copyright

Radio

2007 Schools Wikipedia Selection. Related subjects: Engineering

   Amateur Radio Station with multiple Receivers and Tranceivers
   Amateur Radio Station with multiple Receivers and Tranceivers

   Radio is the wireless transmission of signals, by modulation of
   electromagnetic waves with frequencies below those of visible light.

   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. Information is
   carried by systematically changing ( modulating) some property of the
   radiated waves, such as their amplitude or their frequency. When radio
   waves pass an electrical conductor, the oscillating fields induce an
   alternating current in the conductor. This can be detected and
   transformed into sound or other signals that carry information.

   The word 'radio' is used to describe this phenomenon, and television
   and radio transmissions are classed as radio frequency emissions.

Etymology

   Originally, radio or radioteleography 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 (in Latin "radius" means "spoke of a wheel, beam of
   light, ray"). 'Radio' as a noun is said to have been coined by
   advertising expert Waldo Warren (White 1944). 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 noun 'broadcasting' itself came
   from an agricultural term, meaning 'scattering seeds'.) The American
   term was then adopted by other languages in Europe and Asia, although
   British Commonwealth countries retained the term 'wireless' until the
   mid-20th century. In Japanese, 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'.

   In recent years the term 'wireless' has gained renewed popularity
   through the rapid growth of short range networking, e.g. WLAN
   ('Wireless Local Area Network'), WiFi, Bluetooth as well as mobile
   telephony, e.g. GSM and UMTS. Today, the term 'radio' often refers to
   the actual transceiver device or chip, whereas 'wireless' refers to the
   system and/or method used for radio communication. Hence one talks
   about radio transceivers and Radio Frequency Identification ( RFID),
   but about wireless devices and wireless sensor networks.

Invention

   The identity of the original inventor of radio, at the time called
   wireless telegraphy, is contentious. Development from a laboratory
   demonstration to commercial utility spanned several decades and
   required the efforts of many practitioners. The controversy over who
   invented the radio, with the benefit of hindsight, can be broken down
   as follows:
     * In 1878, David E. Hughes transmitted Morse code by radio at and
       below the Super low frequency range (via a clockwork transmitter).

     * In 1888, Heinrich Hertz produced and measured the Ultra High
       Frequency range (via a sparkgap transmitter).

     * In 1891, Nikola Tesla began wireless research. He developed means
       to reliably produce radio frequencies, publicly demonstrated the
       principles of radio, and transmitted long-distance signals. He
       obtained a U.S. patent for the invention of the radio, as defined
       as "wireless transmission of data."

     * Between 1893 and 1894, Roberto Landell de Moura, a Brazilian priest
       and scientist, conducted experiments. He did not publicise his
       achievement until 1900 but later obtained Brazilian and American
       patents.

     * In 1894 in Kolkata (Calcutta), Sir Jagdish Chandra Bose (J. C.
       Bose) invented the Mercury Coherer (together with the telephone
       receiver), later used by Guglielmo Marconi to receive the radio
       signal in his first transatlantic radio communication over a
       distance of 2000 miles from Poldhu, UK to Newfoundland, St. Johns
       in December 1901. Guglielmo Marconi was celebrated worldwide for
       this achievement, but the fact that the receiver was invented by
       Bose was not well known.

     * Alexander Stepanovich Popov, in 1894, built his first radio
       receiver, which contained a coherer but actually coherer was first
       demonstrated by J.C. Bose. Further refined as a lightning detector,
       he presented it to the Russian Physical and Chemical Society on May
       7, 1895.

     * Guglielmo Marconi was an early radio experimenter. But although
       frequently regarded as the true inventor of the radio, the coherer
       used by him was actually developed by J.C. Bose, who was ignored at
       the time.
     * Reginald Fessenden []and Lee de Forest invented amplitude-modulated
       ( AM) radio, so that more than one station can send signals (as
       opposed to spark-gap radio, where one transmitter covers the entire
       bandwidth of the spectrum).

     * Edwin H. Armstrong invented frequency-modulated ( FM) radio, so
       that an audio signal can avoid "static," that is, interference from
       electrical equipment and atmospherics.

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 wireless 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 (detecting
   devices consisting of tubes filled with iron filings which had been
   invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884) used
   by Guglielmo Marconi and other early experimenters. .

   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.

   The next great invention was the vacuum tube detector, invented by the
   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 (1900-1959) was
   that aircraft used commercial AM radio stations for navigation. This
   continued until the early 1960s when VOR systems finally became
   widespread (though AM stations are still marked on U.S. aviation
   charts). 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. Commercial
   television 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. By 1963 colour television was being regularly
   transmitted commercially, and the first (radio) 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

   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
   from San Jose in 1909 , and became 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 (RAdio Detection And Ranging).

   Today, radio takes many forms, including wireless networks, mobile
   communications of all types, as well as radio broadcasting. 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.

Audio

   A Fisher 500 AM/FM hi-fi receiver from 1959.
   A Fisher 500 AM/FM hi-fi receiver from 1959.

   AM broadcast radio sends music and voice in the Medium Frequency
   (MF—0.300 MHz to 3 MHz) radio spectrum. AM radio uses amplitude
   modulation, in which the amplitude of the transmitted signal is made
   proportional to the sound amplitude captured (transduced) by the
   microphone while the transmitted frequency remains unchanged.
   Transmissions are affected by static and interference because lightning
   and other sources of radio that are transmitting at the same frequency
   add their amplitudes to the original transmitted amplitude. The most
   wattage an AM radio station is allowed to use is 50,000 watts and the
   only stations that can blast out signals this high were grandfathered
   in; these include WJR and CKLW.

   FM broadcast radio sends music and voice with higher fidelity than AM
   radio. In frequency modulation, amplitude variation at the microphone
   cause the transmitter frequency to fluctuate. Because the audio signal
   modulates the frequency and not the amplitude, an FM signal is not
   subject to static and interference in the same way as AM signals. FM is
   transmitted in the Very High Frequency (VHF—30 MHz to 300 MHz) radio
   spectrum. VHF radio waves act more like light, traveling 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. (Use of FM would result in
   stronger stations blocking out reception of weaker stations due to FM's
   capture effect). Aircraft fly high enough that their transmitters can
   be received hundreds of miles (kilometres) away, even though they are
   using VHF.

   Marine voice radios can use AM in the shortwave High Frequency (HF—3
   MHz to 30 MHz) 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 (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. 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 (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, 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

   Mobile phones transmit to a local cell site (transmitter/receiver) that
   ultimately connects to the public switched telephone network ( PSTN)
   through an optic fibre 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 use satellites rather than cell towers to
   communicate. They 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 (4.5 MHz in the NTSC system) away from the
   video carrier. Analog television 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 (MP2) 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 (Very High
   Frequency Omnidirectional Range), systems (used by aircraft), 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 VOR and locate its
   position at the intersection of the two radials, known as a "fix." When
   the VOR station is collocated with DME ( Distance Measuring Equipment),
   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 (Radio Detection And Ranging) 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 (EPIRBs), Emergency
   Locating Transmitters (ELTs) or Personal Locator Beacons (PLBs) 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)

   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 ( Continuous
   Wave), 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 (hams). Strictly, on-off keying of a carrier should be known
   as "Interrupted Continuous Wave" or ICW.

   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. 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). 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. (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 ten below the
   resonant frequency.) 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.

Amateur radio service

   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 licensed
   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 (SSB), AM, digital packet radio and satellite
   repeaters. Some amateur frequencies may be disrupted by power-line
   internet service.

Unlicensed radio services

   Personal radio services such as Citizens' Band Radio, Family Radio
   Service, Multi-Use Radio Service and others exist in North America to
   provide simple, (usually) short range communication for individuals and
   small groups, without the overhead of licensing. Similar services exist
   in other parts of the world. These radio services involve the use of
   handheld or mobile radios better known as "walkie-talkies".

Radio control (RC)

   Radio remote control use of 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 airplanes. Large
   industrial remote-controlled equipment such as cranes and switching
   locomotives now usually use digital radio techniques to ensure safety
   and reliability.

   In Madison Square Garden, at the Electrical Exhibition of 1898, Nikola
   Tesla successfully demonstrated a radio-controlled boat. He was awarded
   U.S. patent No. 613,809 for a "Method of and Apparatus for Controlling
   Mechanism of Moving Vessels or Vehicles."

The electromagnetic spectrum

   Radio waves are a form of electromagnetic radiation, created whenever a
   charged object (in normal radio transmission, an electron) accelerates
   with a frequency that lies in the radio frequency (RF) 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.

                                 Radio spectrum
    ELF   SLF    ULF    VLF     LF      MF      HF     VHF     UHF    SHF   EHF
   3 Hz  30 Hz  300 Hz 3 kHz  30 kHz  300 kHz 3 MHz  30 MHz  300 MHz 3 GHz  30
                                                                            GHz
   30 Hz 300 Hz 3 kHz  30 kHz 300 kHz  3 MHz  30 MHz 300 MHz  3 GHz  30 GHz 300
                                                                            GHz

   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 spectrum and diagram of radio transmission of an audio
   signal. NB The colours used in this diagram of the electromagnetic
   spectrum are for decoration only. They do not correspond to the
   wavelengths and frequencies indicated on the scale.
   Electromagnetic spectrum and diagram of radio transmission of an audio
   signal. NB The colours used in this diagram of the electromagnetic
   spectrum are for decoration only. They do not correspond to the
   wavelengths and frequencies indicated on the scale.
   The Electromagnetic Spectrum
   (Sorted by wavelength, short to long)
   Gamma ray • X-ray • Ultraviolet • Visible spectrum • Infrared •
   Terahertz radiation • Microwave • Radio waves
   Visible (optical) spectrum Violet • Blue • Green • Yellow • Orange •
   Red
   Microwave spectrum W band • V band • K band: K[a] band, K[u] band • X
   band • C band • S band • L band
   Radio spectrum EHF • SHF • UHF • VHF • HF • MF • LF • VLF • ULF • SLF •
   ELF
   Wavelength designations Microwave • Shortwave • Mediumwave • Longwave

Other

   Energy autarkic radio technology consists of a small radio transmitter
   powered by environmental energy (push of a button, temperature
   differences, light, vibrations, etc.). A number of schemes have been
   proposed for Wireless energy transfer. Various plans included
   transmitting power using microwaves, and the technique has been
   demonstrated. (See Microwave power transmission). These schemes
   include, for example, solar power stations in orbit beaming energy down
   to terrestrial users.

   Retrieved from " http://en.wikipedia.org/wiki/Radio"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
