R A D I O
Receive
+ Amplify + Detect + Intelligence + give Output
How radio communication works. Information such as sound is
transformed into an electronic signal which is applied to a transmitter.
The transmitter sends the information through space on a radio wave
(electromagnetic wave). A receiver intercepts some of the radio wave and
extracts the information-bearing electronic signal, which is converted back to
its original form by a transducer such as a speaker.
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 30 kHz to 300 GHz. 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 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
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.
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 mediumwave 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 a carrier wave to transmit a signal. It is another term for Phase
modulation.
Antenna
Rooftop television antennas in Israel. Yagi-Uda
antennas like these six are widely used at VHF and UHF frequencies.
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
applies an oscillating radio frequency electric current 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
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
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
A Crystal Receiver, consisting of an antenna, rheostat, coil, crystal
rectifier, capacitor, headphones and ground connection.
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
|
Name
|
Wavelength
|
||||||
|
less than 0.01 nm
|
more than 10 EHZ
|
100 keV - 300+ GeV
|
|||||
|
0.01 to 10 nm
|
30 PHz - 30 EHZ
|
120 eV to 120 keV
|
|||||
|
10 nm - 400 nm
|
30 EHZ - 790 THz
|
3 eV to 124 eV
|
|||||
|
390 nm - 750 nm
|
790 THz - 405 THz
|
1.7 eV - 3.3 eV
|
|||||
|
750 nm - 1 mm
|
405 THz - 300 GHz
|
||||||
|
1 mm - 1 meter
|
300 GHz - 300 MHz
|
||||||
|
Radio
|
1 mm - km
|
||||||
Radio frequencies occupy the range from a few hertz 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.
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.
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.
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, 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
A Fisher 500 AM/FM hi-fi receiver from
1959.
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.
Bush House,old home of the BBC World Service.
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. 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 (e.g., 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 (500 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 (or kilometres) 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 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. 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.
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 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, 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 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.
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