Radio astronomy is conducted using large radio antennas referred to as radio telescopes, that are either used singularly, or with multiple linked telescopes utilizing the techniques of radio interferometry and aperture synthesis. The use of interferometry allows radio astronomy to achieve high angular resolution,
as the resolving power of an interferometer is set by the distance
between its components, rather than the size of its components.
History
Before
Jansky observed the Milky Way in the 1930s, physicists speculated that
radio waves could be observed from astronomical sources. In the 1860s, James Clerk Maxwell's equations had shown that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. Several attempts were made to detect radio emission from the Sun including an experiment by German astrophysicists Johannes Wilsing and Julius Scheiner in 1896 and a centimeter wave radiation apparatus set up by Oliver Lodge
between 1897-1900. These attempts were unable to detect any emission
due to technical limitations of the instruments. The discovery of the
radio reflecting ionosphere
in 1902, led physicists to conclude that the layer would bounce any
astronomical radio transmission back into space, making them
undetectable.
Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early 1930s. As an engineer with Bell Telephone Laboratories, he was investigating static that interfered with short wave transatlantic voice transmissions. Using a large directional antenna, Jansky noticed that his analog
pen-and-paper recording system kept recording a repeating signal of
unknown origin. Since the signal peaked about every 24 hours, Jansky
originally suspected the source of the interference was the Sun
crossing the view of his directional antenna. Continued analysis showed
that the source was not following the 24-hour daily cycle of the Sun
exactly, but instead repeating on a cycle of 23 hours and 56 minutes.
Jansky discussed the puzzling phenomena with his friend, astrophysicist
and teacher Albert Melvin Skellett, who pointed out that the time
between the signal peaks was the exact length of a sidereal day;
the time it took for "fixed" astronomical objects, such as a star, to
pass in front of the antenna every time the Earth rotated.
By comparing his observations with optical astronomical maps, Jansky
eventually concluded that the radiation source peaked when his antenna
was aimed at the densest part of the Milky Way in the constellation of Sagittarius.
He concluded that since the Sun (and therefore other stars) were not
large emitters of radio noise, the strange radio interference may be
generated by interstellar gas and dust in the galaxy. (Jansky's peak radio source, one of the brightest in the sky, was designated Sagittarius A in the 1950s and, instead of being galactic "gas and dust", was later hypothesized to be emitted by electrons in a strong magnetic field. Current thinking is that these are ions in orbit around a massive Black hole
at the center of the galaxy at a point now designated as Sagitarius A*.
The asterisk indicates that the particles at Sagitarius A are ionized.)
Jansky announced his discovery in 1933. He wanted to investigate the
radio waves from the Milky Way in further detail, but Bell Labs
reassigned him to another project, so he did no further work in the
field of astronomy. His pioneering efforts in the field of radio
astronomy have been recognized by the naming of the fundamental unit of flux density, the jansky (Jy), after him.
Grote Reber
was inspired by Jansky's work, and built a parabolic radio telescope 9m
in diameter in his backyard in 1937. He began by repeating Jansky's
observations, and then conducted the first sky survey in the radio
frequencies. On February 27, 1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun. Later that year George Clark Southworth, at Bell Labs
like Jansky, also detected radiowaves from the sun. Both researchers
were bound by wartime security surrounding radar, so Reber, who was not,
published his 1944 findings first. Several other people independently discovered solar radiowaves, including E. Schott in Denmark and Elizabeth Alexander working on Norfolk Island.
The Robert C. Byrd Green Bank Telescope (GBT) in West Virginia, United States is the world's largest fully steerable radio telescope.
This early research soon branched out into the observation of
other celestial radio sources and interferometry techniques were
pioneered to isolate the angular source of the detected emissions. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis. The radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the Titan) became capable of handling the computationally intensive Fourier transform
inversions required, they used aperture synthesis to create a
'One-Mile' and later a '5 km' effective aperture using the One-Mile and
Ryle telescopes, respectively. They used the Cambridge Interferometer to map the radio sky, producing the famous 2C and 3C surveys of radio sources.
Radio astronomers use different techniques to observe objects in the
radio spectrum. Instruments may simply be pointed at an energetic radio
source to analyze its emission. To “image” a region of the sky in more
detail, multiple overlapping scans can be recorded and pieced together
in a mosaic image. The type of instrument used depends on the strength of the signal and the amount of detail needed.
Observations from the Earth's
surface are limited to wavelengths that can pass through the
atmosphere. At low frequencies, or long wavelengths, transmission is
limited by the ionosphere, which reflects waves with frequencies less than its characteristic plasma frequency. Watervapor
interferes with radio astronomy at higher frequencies, which has led to
building radio observatories that conduct observations at millimeter
wavelengths at very high and dry sites, in order to minimize the water
vapor content in the line of sight. Finally, transmitting devices on
earth may cause radio-frequency interference. Because of this, many radio observatories are built at remote places.
Radio telescopes
An optical image of the galaxy M87 (HST), a radio image of same galaxy using Interferometry (Very Large Array-VLA), and an image of the center section (VLBA) using a Very Long Baseline Array
(Global VLBI) consisting of antennas in the US, Germany, Italy,
Finland, Sweden and Spain. The jet of particles is suspected to be
powered by a black hole in the center of the galaxy.
Radio telescopes may need to be extremely large in order to receive signals with high signal-to-noise ratio. Also since angular resolution is a function of the diameter of the "objective" in proportion to the wavelength of the electromagnetic radiation being observed, radio telescopes have to be much larger in comparison to their optical
counterparts. For example, a 1-meter diameter optical telescope is two
million times bigger than the wavelength of light observed giving it a
resolution of roughly 0.3 arc seconds,
whereas a radio telescope "dish" many times that size may, depending on
the wavelength observed, only be able to resolve an object the size of
the full moon (30 minutes of arc).
Radio interferometry
The difficulty in achieving high resolutions with single radio telescopes led to radio interferometry, developed by British radio astronomer Martin Ryle and Australian engineer, radiophysicist, and radio astronomer Joseph Lade Pawsey and Ruby Payne-Scott
in 1946. Surprisingly the first use of a radio interferometer for an
astronomical observation was carried out by Payne-Scott, Pawsey and Lindsay McCready on 26 January 1946 using a SINGLE converted radar antenna (broadside array) at 200 MHz near Sydney, Australia.
This group used the principle of a sea-cliff interferometer in which
the antenna (formerly a World War II radar) observed the sun at sunrise
with interference arising from the direct radiation from the sun and the
reflected radiation from the sea. With this baseline of almost 200
meters, the authors determined that the solar radiation during the burst
phase was much smaller than the solar disk and arose from a region
associated with a large sunspot group. The Australia group laid out the principles of aperture synthesis in a ground-breaking paper published in 1947. The use of a sea-cliff interferometer
had been demonstrated by numerous groups in Australia, Iran and the UK
during World War II, who had observed interference fringes (the direct
radar return radiation and the reflected signal from the sea) from
incoming aircraft.
The Cambridge group of Ryle and Vonberg observed the sun at
175 MHz for the first time in mid July 1946 with a Michelson
interferometer consisting of two radio antennas with spacings of some
tens of meters up to 240 meters. They showed that the radio radiation
was smaller than 10 arc minutes
in size and also detected circular polarization in the Type I bursts.
Two other groups had also detected circular polarization at about the
same time (David Martyn in Australia and Edward Appleton with James Stanley Hey in the UK).
Modern Radio interferometers consist of widely separated radio telescopes observing the same object that are connected together using coaxial cable, waveguide, optical fiber, or other type of transmission line. This not only increases the total signal collected, it can also be used in a process called Aperture synthesis to vastly increase resolution. This technique works by superposing ("interfering") the signal waves from the different telescopes on the principle that waves that coincide with the same phase
will add to each other while two waves that have opposite phases will
cancel each other out. This creates a combined telescope that is the
size of the antennas furthest apart in the array. In order to produce a
high quality image, a large number of different separations between
different telescopes are required (the projected separation between any
two telescopes as seen from the radio source is called a "baseline") -
as many different baselines as possible are required in order to get a
good quality image. For example, the Very Large Array has 27 telescopes giving 351 independent baselines at once.
Beginning in the 1970s, improvements in the stability of radio
telescope receivers permitted telescopes from all over the world (and
even in Earth orbit) to be combined to perform very-long-baseline interferometry.
Instead of physically connecting the antennas, data received at each
antenna is paired with timing information, usually from a local atomic clock,
and then stored for later analysis on magnetic tape or hard disk. At
that later time, the data is correlated with data from other antennas
similarly recorded, to produce the resulting image. Using this method it
is possible to synthesise an antenna that is effectively the size of
the Earth. The large distances between the telescopes enable very high
angular resolutions to be achieved, much greater in fact than in any
other field of astronomy. At the highest frequencies, synthesised beams
less than 1 milliarcsecond are possible.
The pre-eminent VLBI arrays operating today are the Very Long Baseline Array (with telescopes located across North America) and the European VLBI Network
(telescopes in Europe, China, South Africa and Puerto Rico). Each array
usually operates separately, but occasional projects are observed
together producing increased sensitivity. This is referred to as Global
VLBI. There are also a VLBI networks, operating in Australia and New
Zealand called the LBA (Long Baseline Array), and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).
Since its inception, recording data onto hard media was the only
way to bring the data recorded at each telescope together for later
correlation. However, the availability today of worldwide,
high-bandwidth networks makes it possible to do VLBI in real time. This
technique (referred to as e-VLBI) was originally pioneered in Japan, and
more recently adopted in Australia and in Europe by the EVN (European
VLBI Network) who perform an increasing number of scientific e-VLBI
projects per year.
Astronomical sources
A
radio image of the central region of the Milky Way galaxy. The arrow
indicates a supernova remnant which is the location of a newly
discovered transient, bursting low-frequency radio source GCRT J1745-3009.
Radio astronomy has led to substantial increases in astronomical
knowledge, particularly with the discovery of several classes of new
objects, including pulsars, quasars and radio galaxies.
This is because radio astronomy allows us to see things that are not
detectable in optical astronomy. Such objects represent some of the
most extreme and energetic physical processes in the universe.
The cosmic microwave background radiation
was also first detected using radio telescopes. However, radio
telescopes have also been used to investigate objects much closer to
home, including observations of the Sun and solar activity, and radar mapping of the planets.
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.
Radio systems need a transmitter to modulate (change) some property of the energy produced to impress a signal on it, for example using amplitude modulation or angle modulation (which can be frequency modulation or phase modulation). Radio systems also need an antenna to convert electric currents into radio waves, and radio waves into an electric current. An antenna can be used for both transmitting and receiving. The electrical resonance of tuned circuits in radios allow individual frequencies to be selected. The electromagnetic wave is intercepted by a tuned receiving antenna. A radio receiver receives its input from an antenna
and converts it into a form that is usable for the consumer, such as
sound, pictures, digital data, measurement values, navigational
positions, etc. 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.
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 Ernest Mercadier, Alexander Graham Bell adopted "radiophone" (meaning "radiated sound") as an alternate name for his photophone optical transmission system. However, this invention would not be widely adopted.
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
helped popularize the new word in the United States—in early 1907 he
founded the DeForest Radio Telephone Company, and his letter in the June
22, 1907 Electrical World about the need for legal restrictions
warned that "Radio chaos will certainly be the result until such
stringent regulation is enforced".
The United States Navy would also play a role. Although its translation
of the 1906 Berlin Convention used the terms "wireless telegraph" and
"wireless telegram", by 1912 it began to promote the use of "radio"
instead. The term started to become preferred by the general public in
the 1920s with the introduction of broadcasting. ("Broadcasting" is
based upon an agricultural term meaning roughly "scattering seeds
widely".) British Commonwealth countries continued to commonly use the
term "wireless" until the mid-20th century, though the magazine of the British Broadcasting Corporation in the UK has been called Radio Times since its founding in the early 1920s.
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), Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS
cell phones. Today, the term "radio" specifies the transceiver device
or chip, whereas "wireless" refers to the lack of physical connections;
thus equipment employs embedded radio transceivers, but operates as wireless devices over wireless sensor networks.
Processes
Radio communication. Information such as sound is converted by a transducer such as a microphone to an electrical signal, which modulates a radio wave sent from a transmitter.
A receiver intercepts the radio wave and extracts the
information-bearing electronic signal, which is converted back using
another transducer such as a speaker.
Radio systems used for communication
have the following elements. With more than 100 years of development,
each process is implemented by a wide range of methods, specialised 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 resonantantenna; 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 medium wave broadcast band, but it is used in various radiotelephone services such as the Citizens Band, amateur radio and especially in aviation, due to its ability to be received under very weak signal conditions and its immunity to capture effect, allowing more than one signal to be heard simultaneously.
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. FM has the
"capture effect"
whereby a receiver only receives the strongest signal, even when others
are present. 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 Very high frequency (VHF) radio frequencies for high-fidelitybroadcasts of music and speech. Analog TV sound is also broadcast using FM.
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.
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.
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.
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.
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.
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-frequencybandwidth
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
In 1864 James Clerk Maxwell showed mathematically that electromagnetic waves could propagate through free space. The effects of electromagnetic waves (then-unexplained "action at a distance" sparking behavior) were actually observed before and after Maxwell's work by many inventors and experimenters including George Adams (1780-1784), Luigi Galvani (1791), Peter Samuel Munk (1835), Joseph Henry (1842), Samuel Alfred Varley (1852), Edwin Houston, Elihu Thomson, Thomas Edison (1875) and David Edward Hughes (1878). Edison gave the effect the name "etheric force"
and Hughes detected a spark impulse up to 500 yards (460 m) with a
portable receiver, but none could identify what caused the phenomenon
and it was usually written off as electromagnetic induction. In 1886 Heinrich Rudolf Hertz
noticed the same sparking phenomenon and, in published experiments
(1887-1888), was able to demonstrate the existence of electromagnetic
waves in an experiment confirming Maxwell's theory of electromagnetism.
The discovery of these "Hertzian waves" (radio waves) prompted
many experiments by physicists. An August 1894 lecture by the British
physicist Oliver Lodge,
where he transmitted and received "Hertzian waves" at distances up to
50 meters, was followed up the same year with experiments by Bengali
physicist Jagadish Chandra Bose in extremely high frequency radio microwaveoptics and a year later with the construction of a radio based lightning detector by Russian physicist Alexander Stepanovich Popov. Starting in late 1894, Guglielmo Marconi began pursuing the idea of building a wireless telegraphy
system based on Hertzian waves (radio). Marconi gained a patent on the
system in 1896 and developed it into a commercial communication system
over the next few years.
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 accelerated the development of radio for military communications, and in this era the first vacuum tubes
were applied to radio transmitters and receivers. Electronic
amplification was a key development in changing radio from an
experimental practice by experts into a home appliance. After the war,
commercial radio broadcasting began in the 1920s and became an important
mass medium for entertainment and news. David Sarnoff, an early exponent of broadcast radio, persuaded the Radio Corporation of America to begin an AM broadcasting service which rapidly grew in popularity. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. After the war, the experiments in television that had been interrupted
were resumed, and it also became an important home entertainment
broadcast medium. Stereo FM broadcasting
of radio was taking place from the 1930s onwards in the United States
and displaced AM as the dominant commercial standard by the 1960s, and
by the 1970s in the United Kingdom.
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 important application of radio, in the years just before and
during World War II, was its development and active use for detecting
and locating 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.
Audio
One-way
Bakelite radio at the Bakelite Museum, Orchard Mill, Williton, Somerset, UK.
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. In 1986 KTNN received the last granted 50,000-watt class A 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 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
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.
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.
Two-way
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 shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or Marine VHF radio / 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.
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 GSM or CDMA digital modulation schemes. Satellite phones use satellites rather than cell towers to communicate.
Video
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 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 streamcontainer 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.
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)
2008 Pure One Classic 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 Popov or 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 tubeelectronic 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.
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 experiments. They may also provide
emergency and service assistance in exceptional circumstances. This
contribution has been very beneficial in saving lives in many instances.
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.
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. He was awarded U.S. patent No. 613,809 for a "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles."
