From Wikipedia, the free encyclopedia
This article is about radio as a technology. For other uses, including radio broadcasting as an art form, see Radio (disambiguation).
Radio is the wireless transmission of signals through free space by electromagnetic radiation of a frequency significantly below that of visible light, in the radio frequency range, from about 3 kHz to 300 GHz.[1] These waves are called radio waves. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.
Information, such as sound, is carried by systematically changing (modulating) some property of the radiated waves, 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.
Etymology
The etymology of "radio" or "radiotelegraphy" reveals that it was called "wireless telegraphy", which was shortened to "wireless" in Britain. The prefix radio- in the sense of wireless transmission, was first recorded in the word radioconductor, a description provided by the French physicist Édouard Branly in 1897. It is based on the verb to radiate (in Latin "radius" means "spoke of a wheel, beam of light, ray").
The word "radio" also appears in a 1907 article by Lee De Forest. It was adopted by the United States Navy in 1912, to distinguish radio from several other wireless communication technologies, such as the photophone.
The term 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 widely.") The term
was adopted by other languages in Europe and Asia. British Commonwealth countries continued to commonly use the term "wireless" until the mid-20th century, though the magazine of the BBC in the UK has been called Radio Times ever since it was first published in the early 1920s.
In recent years the more general term "wireless" has gained renewed
popularity through the rapid growth of short-range computer networking,
e.g., Wireless Local Area Network (WLAN), Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS.
Today, the term "radio" specifies the actual type of transceiver device
or chip, whereas "wireless" refers to the lack of physical connections;
one talks about radio transceivers, but another talks about wireless devices and wireless sensor networks.
Processes
Radio systems used for communications
will have the following elements. With more than 100 years of
development, each process is implemented by a wide range of methods,
specialized for different communications purposes.
Transmitter and modulation
See also: Radio transmitter design
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).
Amplitude modulation of a carrier wave
works by varying the strength of the transmitted signal in proportion
to the information being sent. For example, changes in the signal
strength can be used to reflect the sounds to be reproduced by a
speaker, or to specify the light intensity of television pixels. It was
the method used for the first audio radio transmissions, and remains in
use today. "AM" is often used to refer to the medium wave broadcast band (see AM radio).
Frequency modulation varies the frequency
of the carrier. The instantaneous frequency of the carrier is directly
proportional to the instantaneous value of the input signal. Digital
data can be sent by shifting the carrier's frequency among a set of
discrete values, a technique known as frequency-shift keying.
FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM.
Angle modulation alters the instantaneous phase of the carrier wave to transmit a signal. It is another term for phase modulation.
Antenna
Main article: Antenna (radio)
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. An antenna can be used for both transmitting and receiving.
Propagation
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; this is the fundamental limit to the range of
radio communications.
Resonance
Main article: Electrical resonance
See also: LC circuit
Electrical resonance of tuned circuits
in radios allow individual stations to be selected. A resonant circuit
will respond strongly to a particular frequency, and much less so to
differing frequencies. This allows the radio receiver to discriminate
between multiple signals differing in frequency.
Receiver and demodulation
See also: Radio receiver design, Receiver (radio), Radio receiver, Crystal radio, and Communications receiver
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. Radio became more useful
after the invention of electronic devices such as the vacuum tube and later the transistor, which made it possible to amplify weak signals. Today radio systems are used for applications from walkie-talkie children's toys to the control of space vehicles, as well as for broadcasting, and many other applications.
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.[2]
Radio band
Main article: Radio frequency
| Light comparison | |||||||
| Name | Wavelength | Frequency (Hz) | Photon energy (eV) | ||||
|---|---|---|---|---|---|---|---|
| Gamma ray | less than 0.01 nm | more than 10 EHz | 100 keV - 300+ GeV | ||||
| X-Ray | 0.01 to 10 nm | 30 PHz - 30 EHz | 120 eV to 120 keV | ||||
| Ultraviolet | 10 nm - 400 nm | 30 EHz - 790 THz | 3 eV to 124 eV | ||||
| Visible | 390 nm - 750 nm | 790 THz - 405 THz | 1.7 eV - 3.3 eV | ||||
| Infrared | 750 nm - 1 mm | 405 THz - 300 GHz | 1.24 meV - 1.7 eV | ||||
| Microwave | 1 mm - 33 centimeters | 300 GHz - 1000 MHz | 1.24 meV - 3.3 µeV | ||||
| Radio | 1 mm - km | 300 GHz - 3 kHz | 1.24 meV - 12.4 feV | ||||
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.[3] 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.
Communication systems
A radio communication system sends signals by radio.[4] Types of radio communication systems deployed depend on technology, standards, regulations, radio spectrum allocation, user requirements, service positioning, and investment.[5]
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.[6]
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.
History
Main article: History of radio
19th century
Main article: Invention of radio
The meaning and usage of the word "radio" has developed in parallel
with developments within the field of communications and can be seen to
have three distinct phases: electromagnetic waves and experimentation;
wireless communication and technical development; and radio broadcasting
and commercialization. James Clerk Maxwell predicted the propagation of electromagnetic waves (radio waves) (1873) and Heinrich Rudolf Hertz made the first demonstration of transmission of radio waves through free space
(1887) but many individuals—inventors, engineers, developers and
businessmen constructed systems based on their own understanding of
these and other phenomenon, some predating Maxwell and Hertz'
discoveries. Thus "wireless telegraphy" and radio wave based systems can
be attributed to multiple "inventors". Development from a laboratory
demonstration to a commercial entity spanned several decades and
required the efforts of many practitioners.
In 1878, David E. Hughes
noticed that sparks could be heard in a telephone receiver when
experimenting with his carbon microphone. He developed this carbon-based
detector further and eventually could detect signals over a few hundred
yards. He demonstrated his discovery to the Royal Society in 1880, but was told it was merely induction, and therefore abandoned further research.
Experiments were undertaken by Thomas Edison and his employees of Menlo Park. Edison applied in 1885 to the U.S. Patent Office for a patent on an electrostatic coupling system between elevated terminals. The patent was granted as U.S. Patent 465,971 on December 29, 1891. The Marconi Company would later purchase rights to the Edison patent to protect them legally from lawsuits.[7]
In 1884 Temistocle Calzecchi-Onesti at Fermo
in Italy experiments with tubes containing powder and nickel silver
with traces of mercury metal filings and their reactions when conducting
electricity. This would lead to the development of the iron filings
filled coherer, a radio detecting device usually credited to Edouard Branly in 1890.
Between 1886 and 1888 Heinrich Rudolf Hertz
publishes the results of his experiments where he was able to transmit
electromagnetic waves (radio waves) through the air proving Maxwell's
electromagnetic theory.[8][9] Early on after their discovery radio waves were referred to as "Hertzian waves".[10] Between 1890 and 1892 physicists such as John Perry, Frederick Thomas Trouton and William Crookes
proposed electromagnetic or Hertzian waves as a navigation aid or means
of communication with Crookes writing on the possibilities of wireless
telegraphy based on Hertzian waves in 1892.[11]
After learning of Hertz demonstrations of wireless transmission, inventor Nikola Tesla
began developing his own system based on Hertz and Maxwell's ideas,
primarily as a means of wireless lighting and power distribution.[12][13]
Tesla, concluding that Hertz had not demonstrated airborne
electromagnetic waves (radio transmission), went on to develop a system
based on what he thought was the primary conductor, the earth.[14] In 1893 demonstrations of his ideas, in St. Louis, Missouri and at the Franklin Institute in Philadelphia, Tesla proposed this wireless power technology could also incorporate a system for the telecommunication of information.
In a lecture on the work of Hertz, shortly after his death, Professor Oliver Lodge and Alexander Muirhead
demonstrated wireless signaling using Hertzian (radio) waves in the
lecture theater of the Oxford University Museum of Natural History on
August 14, 1894. During the demonstration a radio signal was sent from
the neighboring Clarendon laboratory building, and received by apparatus
in the lecture theater.
Building on the work of Lodge,[15] the Bengali physicist Jagadish Chandra Bose
ignited gunpowder and rang a bell at a distance using millimeter range
wavelength microwaves in a November 1894 public demonstration at Town
Hall of Kolkata,. Bose wrote in a Bengali
essay, Adrisya Alok (Invisible Light), “The invisible light can easily
pass through brick walls, buildings etc. Therefore, messages can be
transmitted by means of it without the mediation of wires.” Bose’s first
scientific paper, “On polarisation of electric rays by
double-refracting crystals” was communicated to the Asiatic Society of
Bengal in May 1895. His second paper was communicated to the Royal
Society of London by Lord Rayleigh in October 1895. In December 1895,
the London journal the Electrician (Vol. 36) published Bose’s paper, “On
a new electro-polariscope”. At that time, the word 'coherer', coined by Lodge, was used in the English-speaking world
for Hertzian wave receivers or detectors. The Electrician readily
commented on Bose’s coherer. (December 1895). The Englishman (18 January
1896) quoted from the Electrician and commented as follows:”Should
Professor Bose succeed in perfecting and patenting his ‘Coherer’, we may
in time see the whole system of coast lighting throughout the navigable
world revolutionised by a Bengali scientist working single handed in
our Presidency College Laboratory.” Bose planned to “perfect his
coherer” but never thought of patenting it.
In 1895, conducting experiments along the lines of Hertz's research, Alexander Stepanovich Popov built his first radio receiver, which contained a coherer. Further refined as a lightning detector,
it was presented to the Russian Physical and Chemical Society on May 7,
1895. A depiction of Popov's lightning detector was printed in the
Journal of the Russian Physical and Chemical Society the same year.
Until recently, mistakenly believed that it was the first description
(publication of the minutes 15/201 of this session — December issue of
the journal RPCS[16]), but in fact the first description of the device was given by Dmitry Aleksandrovich Lachinov in July 1895 in the 2nd edition of his course "Fundamentals of Meteorology and climatology" — the first in Russia.[17][18]
Popov's receiver was created on the improved basis of Lodge's receiver,
and originally intended for reproduction of its experiments.
Marconi's commercialization
In 1894 the young Italian inventor Guglielmo Marconi began working on
the idea of building a commercial wireless telegraphy system based on
the use of Hertzian waves (radio waves), a line of inquiry that he noted
other inventors did not seem to be pursuing.[19] In 1895 he built a radio wave system capable of transmitting signals at long distances (1.5 miles (2.4 km)).[20][21]
Marconi found from his experiments the phenomenon that transmission
range is proportional to the square of antenna height, known as "Marconi's law."[22]
Marconi's experimental apparatus proved to be the first engineering complete, commercially successful radio transmission system.[23][24][25]
In 1896, Marconi was awarded British patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for, the first patent ever issued for a Hertzian wave (radio wave) base wireless telegraphic system.[26] In 1897, he established a radio station on the Isle of Wight, England. Marconi opened his "wireless" factory in the former silk-works at Hall Street, Chelmsford,
England in 1898, employing around 60 people. Shortly after the 1900s,
Marconi held the patent rights for radio. Marconi would go on to be more
successful than any other inventor in his ability to commercialize radio and its associated equipment into a global business.[27]
Marconi knowingly and unknowingly used the scientific and
experimental work of others who were devising their own types of radio
apparatus' around the same time.[27] According to the Proceedings of the United States Naval Institute
in 1899, the Marconi instruments had a "[...] coherer, principle of
which was discovered some twenty years ago, [and was] the only
electrical instrument or device contained in the apparatus that is at
all new."[28]
Some of his subsequent patented refinements (but not his original radio
patent) would be overturned in a US court case in 1935 (upheld in
1943).[29]
20th century
In 1900, Brazilian priest Roberto Landell de Moura transmitted the human voice wirelessly. According the newspaper Jornal do Comercio
(June 10, 1900), he conducted his first public experiment on June 3,
1900, in front of journalists and the General Consul of Great Britain,
C.P. Lupton, in São Paulo,
Brazil, for a distance of approximately 5.0 miles (8 km). The points of
transmission and reception were Alto de Santana and Paulista Avenue.[30]
One year after that experiment, he received his first patent from the
Brazilian government. It was described as "equipment for the purpose of
phonetic transmissions through space, land and water elements at a
distance with or without the use of wires." Four months later, knowing
that his invention had real value, he left Brazil for the United States with the intent of patenting the machine at the US Patent Office in Washington, DC.
Having few resources, he had to rely on friends to push his project.
In spite of great difficulty, three patents were awarded: "The Wave
Transmitter" (October 11, 1904) which is the precursor of today's radio
transceiver; "The Wireless Telephone" and the "Wireless Telegraph", both
dated November 22, 1904.
In June 1912 after the RMS Titanic disaster, due to increased production Marconi opened the world's first purpose-built radio factory at New Street Works, also in Chelmsford, England.
The next advancement was the vacuum tube detector, invented by Westinghouse engineers. On Christmas Eve 1906, Reginald Fessenden used a synchronous rotary-spark transmitter for the first radio program broadcast, from Ocean Bluff-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.[31]
This was, for all intents and purposes, the first transmission of
what is now known as amplitude modulation or AM radio. The first radio
news program was broadcast August 31, 1920 by station 8MK in Detroit, Michigan, which survives today as all-news format station WWJ
under ownership of the CBS network. The first college radio station
began broadcasting on October 14, 1920 from Union College, Schenectady, New York under the personal call letters of Wendell King, an African-American student at the school.[31]
That month 2ADD (renamed WRUC
in 1947), aired what is believed to be the first public entertainment
broadcast in the United States, a series of Thursday night concerts
initially heard within a 100-mile (160 km) radius and later for a
1,000-mile (1,600 km) radius. In November 1920, it aired the first
broadcast of a sporting event.[31][32] At 9 pm on August 27, 1920, Sociedad Radio Argentina aired a live performance of Richard Wagner's opera Parsifal from the Coliseo Theater in downtown Buenos Aires.
Only about twenty homes in the city had receivers to tune in this radio
program. Meanwhile, regular entertainment broadcasts commenced in 1922
from the Marconi Research Centre at Writtle, England.
Sports broadcasting began at this time as well, including the college football on radio broadcast of a 1921 West Virginia vs. Pittsburgh football game.[33]
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 became widespread.[34] 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 1947 AT&T commercialized the Mobile Telephone Service.
From its start in St. Louis in 1946, AT&T then introduced Mobile
Telephone Service to one hundred towns and highway corridors by 1948.
Mobile Telephone Service was a rarity with only 5,000 customers placing
about 30 000 calls each week. Because only
three radio channels were available, only three customers in any given
city could make mobile telephone calls at one time.[35] Mobile Telephone Service was expensive, costing 15 USD per month, plus 0.30 to 0.40 USD per local call, equivalent to about 176 USD per month and 3.50 to 4.75 per call in 2012 USD.[36] The Advanced Mobile Phone System analog mobile cell phone system, developed by Bell Labs, was introduced in the Americas in 1978,[37][38][39]
gave much more capacity. It was the primary analog mobile phone system
in North America (and other locales) through the 1980s and into the
2000s.
In 1954, the Regency company introduced a pocket transistor radio, the TR-1, powered by a "standard 22.5 V Battery." In 1955, the newly formed Sony company introduced its first transistorized radio.[40] It was small enough to fit in a vest
pocket, powered by a small battery. It was durable, because it had no
vacuum tubes to burn out. Over the next 20 years, transistors replaced
tubes almost completely except for high-power transmitters.
By 1963, color television was being broadcast commercially (though
not all broadcasts or programs were in color), 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 launch of the Global Positioning System
(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-defined radio
that can be programmed to be virtually any radio by changing its
software program. 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 it discovered that its
submarine cables had been tapped 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, California in 1909,[41]
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
detection and locating of aircraft and ships by the use of radar (RAdio Detection And Ranging).
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"). Radio was unique among methods of dramatic presentation
in that it used only sound. For more, see radio programming.
Audio
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 emissions on the same frequency
add their amplitudes to the original transmitted amplitude.
In the early part of the 20th century, American AM radio stations
broadcast with powers as high as 500 kW, and some could be heard
worldwide; these stations' transmitters were commandeered for military
use by the US Government during World War II. Currently, the maximum
broadcast power for a civilian AM radio station in the United States and
Canada is 50 kW, and the majority of stations that emit signals this
powerful were grandfathered in (see List of 50 kW AM radio stations in the United States). In 1986 KTNN received the last granted 50,000 watt license. These 50 kW stations are generally called "clear channel" stations (not to be confused with Clear Channel Communications),
because within North America each of these stations has exclusive use
of its broadcast frequency throughout part or all of the broadcast day.
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 the case. 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 nowadays, AM radios limit the audio
bandpass to 3–5 kHz maximum). 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
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 50–200 miles
(80–322 km). 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.
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. Nevertheless, when there were fewer FM stations competing,
this station could be heard near Bloomington, Illinois, US, almost 300
miles (480 km) away.[citation needed]
FM subcarrier
services are secondary signals transmitted in a "piggyback" fashion
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 sub-channel
program might be an alternate foreign-language radio program for various
ethnic groups. Sub-carriers 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 re-tune 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 away, even though they are using VHF.
Marine voice radios can use single sideband voice (SSB) 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. Narrowband FM sacrifices
fidelity to make more channels available within the radio spectrum, by
using a smaller range of radio frequencies, usually with five kHz of
deviation, versus the 75 kHz used by commercial FM broadcasts, and
25 kHz used for TV sound.
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.[42] On an AM radio SSB sounds like ducks quacking, or the adults in a Charlie Brown
cartoon. 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 one of the sidebands. 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 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. Recent developments in Sweden (such as DROPme) allow for the
instant downloading of digital material from a radio broadcast (such as a
song) to a mobile phone.
Satellite phones use satellites rather than cell towers to communicate.
Video
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 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 8VSB modulation in North America (under the ATSC digital television standard), and COFDM modulation elsewhere in the world (using the DVB-T standard). 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 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.
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.
Very High Frequency Omnidirectional Range
(VOR), 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 VORs 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 objects 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
effect to measure wind speeds.
Data (digital radio)
Most new radio systems are digital, including Digital TV, satellite radio, and 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 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.
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 when given an exclusive (non-shared) fixed narrowband
frequency range. Usually the bits are sent in "frames" that repeat. A
special bit pattern is used to locate the beginning of a frame.
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 the 1980s. Bluetooth, most cell phones, and the 802.11b version of Wi-Fi each use various forms of spread spectrum.
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.
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, and induction hobs for cooking.
Amateur radio service
Amateur radio, also known as "ham radio", is a hobby in which enthusiasts are licensed to communicate on a number of bands in the radio frequency spectrum
non-commercially and for their own enjoyment. They may also provide
emergency and public service assistance. This has been very beneficial
in emergencies, saving lives in many instances.[43]
Radio amateurs use a variety of modes, including nostalgic ones like Morse code and experimental ones like Low-Frequency Experimental Radio. 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 illegally by power-line internet 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.
Wi-Fi also operates in unlicensed radio bands and is very widely used to network computers.
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.[44][45]
Radio control (RC)
Radio remote controls use 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.[46] He was awarded U.S. patent No. 613,809 for a "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles."[47]
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