RADIO is the technology of using radio waves to carry information, such as sound, by systematically modulating properties of electromagnetic energy waves transmitted through space, such as their amplitude , frequency , phase , or pulse width . When radio waves strike an electrical conductor , the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.
A radio communication system sends signals by radio. The radio equipment involved in communication systems includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the receiver in the case of a voice-communication system.
* 1 Etymology
* 2 Processes
* 3 Communication systems * 4 History
* 5 Uses of radio
* 5.1 Audio
* 5.1.1 One-way * 5.1.2 Two-way
* 5.2 Telephony
* 5.3 Video
* 5.4 Navigation
* 6 See also * 7 Notes * 8 References * 9 External links
The term "radio" is derived from the Latin word "radius", meaning
"spoke of a wheel, beam of light, ray". It was first applied to
communications in 1881 when, at the suggestion of French scientist
Alexander Graham Bell
Following Heinrich Hertz 's establishment of the existence of electromagnetic radiation in the late 1880s, a variety of terms were initially used for the phenomenon, with early descriptions of the radiation itself including "Hertzian waves", "electric waves", and "ether waves", while phrases describing its use in communications included "spark telegraphy", "space telegraphy", "aerography" and, eventually and most commonly, "wireless telegraphy". However, "wireless" included a broad variety of related electronic technologies, including electrostatic induction , electromagnetic induction and aquatic and earth conduction , so there was a need for a more precise term referring exclusively to electromagnetic radiation.
The first use of radio- in conjunction with electromagnetic radiation appears to have been by French physicist Édouard Branly , who in 1890 developed a version of a coherer receiver he called a radio-conducteur. The radio- prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe, for example, in early 1898 the British publication The Practical Engineer included a reference to "the radiotelegraph" and "radiotelegraphy", while the French text of both the 1903 and 1906 Berlin Radiotelegraphic Conventions includes the phrases radiotélégraphique and radiotélégrammes.
The use of "radio" as a standalone word dates back to at least December 30, 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions". This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included a Service Regulation specifying that "Radiotelegrams shall show in the preamble that the service is 'Radio'".
The switch to "radio" in place of "wireless" took place slowly and
unevenly in the English-speaking world.
Lee de Forest
In recent years the more general term "wireless" has gained renewed
popularity, even for devices using electromagnetic radiation, through
the rapid growth of short-range computer networking, e.g., Wireless
Local Area Network (WLAN) ,
TRANSMITTER AND MODULATION
Each system contains a transmitter , This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna ; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space (sometimes with a particular polarization ). An audio signal (top) may be carried by an AM or FM radio wave.
Angle modulation alters the instantaneous phase of the carrier wave to transmit a signal. It may be either FM or phase modulation (PM).
An antenna (or aerial) is an electrical device which converts electric currents into radio waves , and vice versa. It is usually used with a radio transmitter or radio receiver . In transmission , a radio transmitter supplies an electric current oscillating at radio frequency (i.e. high frequency AC) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified . Some antennas can be used for both transmitting and receiving, even simultaneously, depending on the connected equipment.
Main article: Radio propagation
Once generated, electromagnetic waves travel through space either directly, or have their path altered by reflection , refraction or diffraction . The intensity of the waves diminishes due to geometric dispersion (the inverse-square law ); some energy may also be absorbed by the intervening medium in some cases. Noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used. If the magnitude of the noise is large enough, the desired signal will no longer be discernible; the signal-to-noise ratio is the fundamental limit to the range of radio communications.
RECEIVER AND DEMODULATION
The electromagnetic wave is intercepted by a tuned receiving antenna ; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated , which is conversion to a usable signal form by a detector sub-system. The receiver is "tuned " to respond preferentially to the desired signals, and reject undesired signals.
Early radio systems relied entirely on the energy collected by an
antenna to produce signals for the operator.
A radio receiver receives its input from an antenna , uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.
Main article: Radio frequency
NAME FREQUENCY (HZ) (WAVELENGTH) PHOTON ENERGY (EV)
Gamma ray > 30 EHz (0.01 nm) 124 keV - 300+ GeV
30 EHz - 30 PHz (0.01 nm - 10 nm) 124 eV to 120 keV
30 PHz - 750 THz (10 nm - 400 nm) 3.1 eV to 124 eV
750 THz - 428.5 THz (400 nm - 700 nm) 1.7 eV - 3.1 eV
428.5 THz - 300 GHz (700 nm - 1 mm) 1.24 meV - 1.7 eV
300 GHz - 300 MHz (1 mm - 1 m) 1.24 µeV - 1.24 meV
300 MHz - 3 kHz (1 m - 100 km) 12.4 feV - 1.24 meV
Radio frequencies occupy the range from a 3 kHz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum. Other types of electromagnetic radiation, with frequencies above the RF range, are 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 .
A radio communication system sends signals by radio. Types of radio communication systems deployed depend on technology , standards , regulations , radio spectrum allocation , user requirements , service positioning , and investment .
The radio equipment involved in communication systems includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the receiver in the case of a voice-communication system. The power consumed in a transmitting station varies depending on the distance of communication and the transmission conditions. The power received at the receiving station is usually only a tiny fraction of the transmitter's output, since communication depends on receiving the information , not the energy , that was transmitted.
Classical radio communications systems use frequency-division multiplexing (FDM) as a strategy to split up and share the available radio-frequency bandwidth for use by different parties' communications concurrently. Modern radio communication systems include those that divide up a radio-frequency band by time-division multiplexing (TDM) and code-division multiplexing (CDM) as alternatives to the classical FDM strategy. These systems offer different tradeoffs in supporting multiple users, beyond the FDM strategy that was ideal for broadcast radio but less so for applications such as mobile telephony .
A radio communication system may send information only one way. For example, in broadcasting a single transmitter sends signals to many receivers. Two stations may take turns sending and receiving, using a single radio frequency; this is called "simplex." By using two radio frequencies, two stations may continuously and concurrently send and receive signals - this is called "duplex " operation.
Main article: History of radio
James Clerk Maxwell
The discovery of these "Hertzian waves" (radio waves) prompted many
experiments by physicists. An August 1894 lecture by the British
Oliver Lodge , where he transmitted and received "Hertzian
waves" at distances up to 50 meters, was followed up the same year
with experiments by Indian physicist
Jagadish Chandra Bose
Early 20th century radio systems transmitted messages by continuous
wave code only. Early attempts at developing a system of amplitude
modulation for voice and music were demonstrated in 1900 and 1906, but
had little success.
World War I
USES OF RADIO
For a broader coverage related to this topic, see
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
Today, radio takes many forms, including wireless networks and 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 (the era from the late 1920s to the mid-1950s
is commonly called radio's "Golden Age").
In the early part of the 20th century, American
FM broadcast radio sends music and voice with less noise than AM
radio. It is often mistakenly thought that FM is higher fidelity than
AM, but that is not true. AM is capable of the same audio bandwidth
that FM employs. AM receivers typically use narrower filters in the
receiver to recover the signal with less noise.
AM stereo receivers
can reproduce the same audio bandwidth that FM does due to the wider
filter used in an
AM stereo receiver, but today, AM radios limit the
audio bandpass to 3–5 kHz. In frequency modulation, amplitude
variation at the microphone causes 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. Due to its need for a wider bandwidth,
FM is transmitted in the Very High
VHF radio waves act more like light, traveling in straight lines;
hence the reception range is generally limited to about 50–200 miles
(80–322 km). During unusual upper atmospheric conditions, FM signals
are occasionally reflected back towards the
High power is useful in penetrating buildings, diffracting around hills, and refracting in the dense atmosphere near the horizon for some distance beyond the horizon. Consequently, 100,000-watt FM stations can regularly be heard up to 100 miles (160 km) away, and farther, 150 miles (240 km), if there are no competing signals. A few old, "grandfathered" stations do not conform to these power rules. WBCT-FM (93.7) in Grand Rapids, Michigan , US, runs 320,000 watts ERP, and can increase to 500,000 watts ERP by the terms of its original license. Such a huge power level does not usually help to increase range as much as one might expect, because VHF frequencies travel in nearly straight lines over the horizon and off into space.
FM subcarrier services are secondary signals transmitted in a
"piggyback" fashion along with the main program.
Aviation voice radios use Aircraft band 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 away, even though they are using VHF. Degen DE1103, an advanced world mini-receiver with single sideband modulation and dual conversion
Marine voice radios can use single sideband voice (SSB) in the
Government, police, fire and commercial voice services also use
narrowband FM on special frequencies. Early police radios used AM
receivers to receive one-way dispatches. 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
Terrestrial Trunked Radio
Mobile phones transmit to a local cell site (transmitter/receiver)
that ultimately connects to the public switched telephone network
(PSTN ) 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 either
Analog television sends the picture as AM and the sound as AM or FM,
with the sound carrier a fixed frequency (4.5 MHz in the
Main article: Radio 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.
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.
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
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 effect to measure wind speeds.
DATA (DIGITAL RADIO)
2008 Pure One Classic digital radio
Most new radio systems are digital, including Digital TV , satellite
radio, and Digital Audio
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 or on-off keying (OOK).
Radioteletype equipment usually operates on short-wave (HF) and is much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones using frequency-shift keying . Groups of five or seven bits become a character printed by a teleprinter. From about 1925 to 1975, radioteletype 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.
Communication systems that limit themselves to a fixed narrowband
frequency range are vulnerable to jamming . A variety of
jamming-resistant spread spectrum techniques were initially developed
for military use, most famously for Global Positioning System
satellite transmissions. Commercial use of spread spectrum began in
Bluetooth , most cell phones, and the 802.11b version of
Systems that need reliability, or that share their frequency with other services, may use "coded 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 Wi-Fi, some cell phones, Digital Radio Mondiale , Eureka 147 , and many other local area network, digital TV and radio standards.
Main article: Radio-frequency 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.
AMATEUR RADIO SERVICE
UNLICENSED RADIO SERVICES
Unlicensed, government-authorized personal radio services such as Citizens\' band radio in Australia, most of the Americas, and Europe, and Family Radio Service and Multi-Use Radio Service in North America exist 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 units.
Free radio stations, sometimes called pirate radio or "clandestine" stations, are unauthorized, unlicensed, illegal broadcasting stations. These are often low power transmitters operated on sporadic schedules by hobbyists, community activists, or political and cultural dissidents. Some pirate stations operating offshore in parts of Europe and the United Kingdom more closely resembled legal stations, maintaining regular schedules, using high power, and selling commercial advertising time.
RADIO CONTROL (RC)
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."
* ^ While the term 'radio-' is actually the combining form of radiant (e.g., radioactive , radiotherapy ), the process that was originally called radiotelegraphy has become so common that it is nearly always called just 'radio' and the associated electromagnetic waves are called radio waves . In practice, radio frequencies are significantly below that of visible light from about 7003300000000000000♠3 kHz to 300 GHz.
* ^ Dictionary of