are a type of electromagnetic radiation with wavelengths
in the electromagnetic spectrum longer than infrared light. Radio
waves have frequencies as high as 300
to as low as
3 kHz, though some definitions describe waves above
300 MHz or 3
as microwaves, or include waves of any
lower frequency. At 300 GHz, the corresponding wavelength is
1 mm (0.039 in), and at 3 kHz is 100 km
(62 mi). Like all other electromagnetic waves, they travel at the
speed of light. Naturally occurring radio waves are generated by
lightning, or by astronomical objects.
Artificially generated radio waves are used for fixed and mobile radio
communication, broadcasting, radar and other navigation systems,
communications satellites, computer networks and many other
are generated by radio transmitters and
received by radio receivers. Different frequencies of radio waves have
different propagation characteristics in the Earth's atmosphere; long
waves can diffract around obstacles like mountains and follow the
contour of the earth (ground waves), shorter waves can reflect off the
ionosphere and return to earth beyond the horizon (skywaves), while
much shorter wavelengths bend or diffract very little and travel on a
line of sight, so their propagation distances are limited to the
To prevent interference between different users, the artificial
generation and use of radio waves is strictly regulated by law,
coordinated by an international body called the International
Telecommunications Union (ITU), which defines radio waves as
"electromagnetic waves of frequencies arbitrarily lower than
3 000 GHz, propagated in space without artificial guide".
The radio spectrum is divided into a number of radio bands on the
basis of frequency, allocated to different uses.
Diagram of the electric fields (E) and magnetic fields (H) of radio
waves emitted by a monopole radio transmitting antenna (small dark
vertical line in the center). The E and H fields are perpendicular as
implied by the phase diagram in the lower right.
1 Discovery and exploitation
3 Speed, wavelength, and frequency
4 Radio communication
5 Biological and environmental effects
6 See also
Discovery and exploitation
Main article: History of radio
Rough plot of Earth's atmospheric transmittance (or opacity) to
various wavelengths of electromagnetic radiation, including radio
Radio waves were first predicted by mathematical work done in 1867 by
Scottish mathematical physicist James Clerk Maxwell. Maxwell
noticed wavelike properties of light and similarities in electrical
and magnetic observations. His mathematical theory, now called
Maxwell's equations, described light waves and radio waves as waves of
electromagnetism that travel in space, radiated by a charged particle
as it undergoes acceleration. In 1887,
Heinrich Hertz demonstrated the
reality of Maxwell's electromagnetic waves by experimentally
generating radio waves in his laboratory, showing that they
exhibited the same wave properties as light: standing waves,
refraction, diffraction, and polarization. Radio waves, originally
called "Hertzian waves", were first used for communication in the
mid 1890s by Guglielmo Marconi, who developed the first practical
radio transmitters and receivers. The modern term "radio wave"
replaced the original name "Hertzian wave" around 1912.
Main article: Radio propagation
The study of radio propagation, how radio waves move in free space and
over the surface of the Earth, is vitally important in the design of
practical radio systems.
Radio waves passing through different
environments experience reflection, refraction, polarization,
diffraction, and absorption. Different frequencies experience
different combinations of these phenomena in the Earth's atmosphere,
making certain radio bands more useful for specific purposes than
others. Practical radio systems mainly use three different techniques
of radio propagation to communicate:
Line of sight: This refers to radio waves that travel in a straight
line from the transmitting antenna to the receiving antenna. It does
not necessarily require a cleared sight path; at lower frequencies
radio waves can pass through buildings, foliage and other
obstructions. This is the only method of propagation possible at
frequencies above 30 MHz. On the surface of the Earth, line of
sight propagation is limited by the visual horizon to about 64 km
(40 mi). This is the method used by cell phones, FM and
television broadcasting and radar. By using dish antennas to transmit
beams of microwaves, point-to-point microwave relay links transmit
telephone and television signals over long distances up to the visual
horizon. Ground stations can communicate with satellites and
spacecraft billions of miles from Earth.
Radio waves can reach points beyond the
line-of-sight by diffraction and reflection.
Diffraction allows a
radio wave to bend around obstructions such as a building edge, a
vehicle, or a turn in a hall.
Radio waves also reflect from surfaces
such as walls, floors, ceilings, vehicles and the ground. These
propagation methods occur in short range radio communication systems
such as cell phones, cordless phones, walkie-talkies, and wireless
networks. A drawback of this mode is multipath propagation, in which
radio waves travel from the transmitting to the receiving antenna via
multiple paths. The waves interfere, often causing fading and other
Ground waves: At lower frequencies below 2 MHz, in the medium
wave and longwave bands, due to diffraction vertically polarized radio
waves can bend over hills and mountains, and propagate beyond the
horizon, traveling as surface waves which follow the contour of the
Earth. This allows mediumwave and longwave broadcasting stations to
have coverage areas beyond the horizon, out to hundreds of miles. As
the frequency drops, the losses decrease and the achievable range
increases. Military very low frequency (VLF) and extremely low
frequency (ELF) communication systems can communicate over most of the
Earth, and with submarines hundreds of feet underwater.
Skywaves: At medium wave and shortwave wavelengths, radio waves
reflect off conductive layers of charged particles (ions) in a part of
the atmosphere called the ionosphere. So radio waves directed at an
angle into the sky can return to Earth beyond the horizon; this is
called "skip" or "skywave" propagation. By using multiple skips
communication at intercontinental distances can be achieved. Skywave
propagation is variable and dependent on atmospheric conditions; it is
most reliable at night and in the winter. Widely used during the first
half of the 20th century, due to its unreliability skywave
communication has mostly been abandoned. Remaining uses are by
military over-the-horizon (OTH) radar systems, by some automated
systems, by radio amateurs, and by shortwave broadcasting stations to
broadcast to other countries.
Speed, wavelength, and frequency
Radio waves in vacuum travel at the speed of light. When passing
through a material medium, they are slowed according to that object's
permeability and permittivity. Air is thin enough that in the Earth's
atmosphere radio waves travel very close to the speed of light.
The wavelength is the distance from one peak of the wave's electric
field (wave's peak/crest) to the next, and is inversely proportional
to the frequency of the wave. The distance a radio wave travels in one
second, in a vacuum, is 299,792,458 meters (983,571,056 ft) which
is the wavelength of a 1 hertz radio signal. A 1 megahertz radio
signal has a wavelength of 299.8 meters (984 ft).
Main article: Radio communication
In radio communication systems, information is carried across space
using radio waves. At the sending end, the information to be sent, in
the form of a time-varying electrical signal, is applied to a radio
transmitter. The information signal can be an audio signal
representing sound from a microphone, a video signal representing
moving images from a video camera, or a digital signal representing
data from a computer. In the transmitter, an electronic oscillator
generates an alternating current oscillating at a radio frequency,
called the carrier because it serves to "carry" the information
through the air. The information signal is used to modulate the
carrier, altering some aspect of it, "piggybacking" the information on
the carrier. The modulated carrier is amplified and applied to an
antenna. The oscillating current pushes the electrons in the antenna
back and forth, creating oscillating electric and magnetic fields,
which radiate the energy away from the antenna as radio waves. The
radio waves carry the information to the receiver location.
At the receiver, the oscillating electric and magnetic fields of the
incoming radio wave push the electrons in the receiving antenna back
and forth, creating a tiny oscillating voltage which is a weaker
replica of the current in the transmitting antenna. This voltage
is applied to the radio receiver, which extracts the information
signal. The receiver first uses a bandpass filter to separate the
desired radio station's radio signal from all the other radio signals
picked up by the antenna, then amplifies the signal so it is stronger,
then finally extracts the information-bearing modulation signal in a
demodulator. The recovered signal is sent to a loudspeaker or earphone
to produce sound, or a television display screen to produce a visible
image, or other devices. A digital data signal is applied to a
computer or microprocessor, which interacts with a human user.
The radio waves from many transmitters pass through the air
simultaneously without interfering with each other. They can be
separated in the receiver because each transmitter's radio waves
oscillate at a different rate, in other words each transmitter has a
different frequency, measured in kilohertz (kHz), megahertz (MHz) or
gigahertz (GHz). The bandpass filter in the receiver consists of a
tuned circuit which acts like a resonator, similarly to a tuning
fork. It has a natural resonant frequency at which it oscillates.
The resonant frequency is set equal to the frequency of the desired
radio station. The oscillating radio signal from the desired station
causes the tuned circuit to oscillate in sympathy, and it passes the
signal on to the rest of the receiver. Radio signals at other
frequencies are blocked by the tuned circuit and not passed on.
Biological and environmental effects
Radio waves are nonionizing radiation, which means they do not have
enough energy to separate electrons from atoms or molecules, ionizing
them, or break chemical bonds, causing chemical reactions or DNA
damage. The main effect of absorption of radio waves by materials is
to heat them, similarly to the infrared waves radiated by sources of
heat such as a space heater or wood fire. The oscillating electric
field of the wave causes polar molecules to vibrate back and forth,
increasing the temperature; this is how a microwave oven cooks food.
However, unlike infrared waves, which are mainly absorbed at the
surface of objects and cause surface heating, radio waves are able to
penetrate the surface and deposit their energy inside materials and
biological tissues. The depth to which radio waves penetrate decreases
with their frequency, and also depends on the material's resistivity
and permittivity; it is given by a parameter called the skin depth of
the material, which is the depth within which 63% of the energy is
deposited. For example, the 2.45
GHz radio waves (microwaves) in
a microwave oven penetrate most foods approximately 2.5 to 3.8 cm
(1 to 1.5 inches).
Radio waves have been applied to the body for 100
years in the medical therapy of diathermy for deep heating of body
tissue, to promote increased blood flow and healing. More recently
they have been used to create higher temperatures in hyperthermia
treatment, to kill cancer cells. Looking into a source of radio waves
at close range, such as the waveguide of a working radio transmitter,
can cause damage to the lens of the eye by heating. A strong enough
beam of radio waves can penetrate the eye and heat the lens enough to
Since the heating effect is in principle no different from other
sources of heat, most research into possible health hazards of
exposure to radio waves has focused on "nonthermal" effects; whether
radio waves have any effect on tissues besides that caused by heating.
Electromagnetic radiation has been classified by the International
Agency for Research on Cancer (IARC) as "Possibly carcinogenic to
Radio waves can be shielded against by a conductive metal sheet or
screen, an enclosure of sheet or screen is called a Faraday cage. A
metal screen shields against radio waves as well as a solid sheet as
long as the holes in the screen are smaller than about 1/20 of
wavelength of the waves.
Look up radio wave in Wiktionary, the free dictionary.
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^ Jones, Graham A.; Layer, David H.; Osenkowsky, Thomas G. (2013).
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^ ITU Radio Regulations, Chapter I, Section I, General terms –
Article 1.5, definition: radio waves or hertzian waves
^ Harman, Peter Michael (1998). The natural philosophy of James Clerk
Maxwell. Cambridge, England: Cambridge University Press. p. 6.
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University Library Historical Monographs Collection. Reprinted by
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Karl Rawer: "Wave Propagation in the Ionosphere". Kluwer, Dordrecht
1993. ISBN 0-7923-0775-5
Radio spectrum (ITU)
3 Hz/100 Mm
30 Hz/10 Mm
30 Hz/10 Mm
300 Hz/1 Mm
300 Hz/1 Mm
3 kHz/100 km
3 kHz/100 km
30 kHz/10 km
30 kHz/10 km
300 kHz/1 km
300 kHz/1 km
3 MHz/100 m
3 MHz/100 m
30 MHz/10 m
30 MHz/10 m
300 MHz/1 m
300 MHz/1 m
3 GHz/100 mm
3 GHz/100 mm
30 GHz/10 mm
30 GHz/10 mm
300 GHz/1 mm
300 GHz/1 mm
3 THz/0.1 mm
← higher frequencies longer
Radiation (physics and health)
Acoustic radiation force
Earth's energy budget
Black body radiation
Cosmic background radiation
Acute radiation syndrome
Electromagnetic radiation and health
Lasers and aviation safety
Mobile phone radiation and health
Radioactivity in the life sciences
Biological dose units and quantities
Wireless electronic devices and health
List of civilian radiation accidents
1996 Costa Rica accident
1987 Goiânia accident
1984 Moroccan accident
1990 Zaragoza accident
See also: the categories
Radiation effects, Radioactivity,