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Radio




Radio is a technology that allows for the transmission of sound or other
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, cosmic 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.

Discovery

The theoretical basis of the propagation of electromagnetic waves was first
described 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, and discovering that the electromagnetic
equations could be reformulated into a partial differential equation called
the wave equation.

On Christmas Eve, 1906, using his heterodyne principle, Reginald Fessenden
transmitted the first radio 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.

Invention

The identity of the original inventor of radio, at the time called wireless
telegraphy, is contentious.

In St. Louis, Missouri, Nikola Tesla made the first public demonstration of
radio communication in 1893. 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.

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 the USA, some key developments in radio's early history were created and
patented in 1897 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.

Uses of Radio

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
        o The oldest form of audio broadcast was marine radio telegraphy,
          now no longer used. 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.
        o 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.
        o 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). 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, and travel in
          straight lines that do not bend around the Earth.
        o FM Sub-band services transmit digital data, such as station
          identification, the current song's name, web addresses, or stock
          quotes on unused space in an FM station's allocation. In some
          countries, FM radios automatically retune themselves to the same
          channel in a different district by using sub-bands.
        o Marine and aviation voice radios use VHF AM. This is good for
          aircraft and boats because the antennas are lightweight. Aircraft
          are often so high that their radios can see hundreds of miles,
          even though they are using VHF.
        o 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.
        o 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.
        o TETRA, Terrestial Trunked Radio is a digital cell phone system for
          military, police and ambulances.

   * Telephony
        o 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.
        o 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
        o Television sends the picture as AM, and the sound as FM, on the
          same radio signal.
        o 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.

   * Navigation
        o 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.
        o Loran systems also used time-of-flight radio signals, but from
          radio stations on the ground.
        o 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 ransmitter pulses. An aircraft can get readings
          from two VORs, and locate its position at the intersection of the
          two beams.
        o 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
        o 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.
        o 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.
        o 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.
        o 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.
        o 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.
        o 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
        o emergency position indicating radio 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
        o 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.
        o 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.
        o 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.
        o Aircraft use a 1200 Baud radioteletype service over VHF to send
          their ID, altitude and position, and get gate and
          connecting-flight data.

   * Heating
        o 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
        o Tractor beams: Radio waves exert small electrostatic and magnetic
          forces. These are enough to perform station-keeping in
          microgravity environments.
        o 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
        o 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.

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