Type of electromagnetic radiation
Animation of a
half-wave dipole
antenna
radiating radio waves, showing the
electric field
lines. The antenna in the center is two vertical metal rods connected to a
radio transmitter
(not shown). The transmitter applies an
alternating electric current
to the rods, which charges them alternately
positive
(+) and
negative
(?). Loops of electric field leave the antenna and travel away at the
speed of light
; these are the radio waves. In this animation the action is shown slowed down tremendously.
Radio waves
are a type of
electromagnetic radiation
with the lowest
frequencies
and the longest
wavelengths
in the
electromagnetic spectrum
, typically with frequencies below 300
gigahertz
(GHz) and wavelengths greater than
1 millimeter (
3
⁄
64
inch), about the diameter of a grain of rice.
Like all electromagnetic waves, radio waves in a vacuum travel at the
speed of light
, and in the Earth's atmosphere at a slightly slower speed. Radio waves are generated by
charged particles
undergoing
acceleration
, such as time-varying
electric currents
.
[1]
Naturally occurring radio waves are emitted by
lightning
and
astronomical objects
, and are part of the
blackbody radiation
emitted by all warm objects.
Radio waves are generated artificially by an electronic device called a
transmitter
, which is connected to an
antenna
which radiates the waves. They are received by another antenna connected to a
radio receiver
, which processes the received signal. Radio waves are very widely used in modern technology for fixed and mobile
radio communication
,
broadcasting
,
radar
and
radio navigation
systems,
communications satellites
,
wireless computer networks
and many other applications. 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 visual horizon.
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 Telecommunication Union
(ITU), which defines radio waves as "
electromagnetic waves
of
frequencies
arbitrarily lower than 3,000
GHz
, propagated in space without artificial guide".
[2]
The
radio spectrum
is divided into a number of radio bands on the basis of frequency, allocated to different uses. Higher-frequency, shorter-wavelength radio waves are called
microwaves
.
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.
Discovery and exploitation
[
edit
]
Radio waves were first predicted by the theory of
electromagnetism
proposed in 1867 by Scottish mathematical physicist
James Clerk Maxwell
.
[3]
His mathematical theory, now called
Maxwell's equations
, predicted that a coupled
electric
and
magnetic field
could travel through space as an "
electromagnetic wave
". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicist
Heinrich Hertz
demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory,
[4]
showing that they exhibited the same wave properties as light:
standing waves
,
refraction
,
diffraction
, and
polarization
. Italian inventor
Guglielmo Marconi
developed the first practical radio transmitters and receivers around 1894?1895. He received the 1909
Nobel Prize in physics
for his radio work. Radio communication began to be used commercially around 1900. The modern term "
radio wave
" replaced the original name "
Hertzian wave
" around 1912.
Generation and reception
[
edit
]
Animated diagram of a
half-wave dipole
antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver
R
. The
electric field
(
E
, green arrows
) of the incoming wave pushes the
electrons
in the rods back and forth, charging the ends alternately positive
(+)
and negative
(?)
. Since the length of the antenna is one half the
wavelength
of the wave, the oscillating field induces
standing waves
of voltage (
V
, represented by red band
) and current in the rods. The oscillating currents (black arrows) flow down the transmission line and through the receiver (represented by the resistance
R
).
Radio waves are radiated by
charged particles
when they are
accelerated
. Natural sources of radio waves include
radio noise
produced by
lightning
and other natural processes in the Earth's atmosphere, and
astronomical radio sources
in space such as the Sun, galaxies and nebulas. All warm objects radiate high frequency radio waves (
microwaves
) as part of their
black body radiation
.
Radio waves are produced artificially by time-varying
electric currents
, consisting of
electrons
flowing back and forth in a specially-shaped metal conductor called an
antenna
. An electronic device called a
radio transmitter
applies oscillating electric current to the antenna, and the antenna radiates the power as radio waves. Radio waves are received by another antenna attached to a
radio receiver
. When radio waves strike the receiving antenna they push the electrons in the metal back and forth, creating tiny oscillating currents which are detected by the receiver.
From
quantum mechanics
, like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged
elementary particles
called
photons
.
[5]
In an antenna transmitting radio waves, the electrons in the antenna emit the energy in discrete packets called radio photons, while in a receiving antenna the electrons absorb the energy as radio photons. An antenna is a
coherent
emitter of photons, like a
laser
, so the radio photons are all
in phase
.
[6]
[5]
However, from
Planck's relation
the energy of individual radio photons is extremely small,
[5]
from 10
?22
to 10
?30
joules
. So the antenna of even a very low power transmitter emits enormous numbers of photons per second. Therefore, except for certain
molecular electron transition
processes such as atoms in a
maser
emitting microwave photons, radio wave emission and absorption is usually regarded as a continuous
classical
process, governed by
Maxwell's equations
.
Properties
[
edit
]
Radio waves in a vacuum travel at the
speed of light
.
[7]
[8]
When passing through a material medium, they are slowed depending on the medium'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 (crest) of the wave's electric field to the next, and is inversely proportional to the
frequency
of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is
![{\displaystyle \lambda ={\frac {\;c\;}{f}}~,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/b1b667e5aab0c5a1f20ed30f894e88c5ba7ae631)
where
![{\displaystyle c\approx 299.79\times 10^{6}{\text{ m/s}}~.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/60d4084f9f39cd35c6f84bbd5b26a751577feab0)
Equivalently,
the distance a radio wave travels in a vacuum, in one second, is 299,792,458 meters (983,571,056 ft), which is the wavelength of a 1
Hertz
radio signal. A 1
megahertz
radio wave (mid-
AM band
) has a wavelength of 299.79 meters (983.6 ft).
Polarization
[
edit
]
Like other electromagnetic waves, a radio wave has a property called
polarization
, which is defined as the direction of the wave's oscillating
electric field
perpendicular to the direction of motion. A plane polarized radio wave has an electric field which oscillates in a plane perpendicular to the direction of motion. In a
horizontally polarized
radio wave the electric field oscillates in a horizontal direction. In a
vertically polarized
wave the electric field oscillates in a vertical direction. In a
circularly polarized
wave the electric field at any point rotates about the direction of travel, once per cycle. A
right circularly polarized
wave rotates in a
right hand
sense about the direction of travel, while a
left circularly polarized
wave rotates in the opposite sense.
[9]
: p.21
The wave's
magnetic field
is perpendicular to the electric field, and the electric and magnetic field are oriented in a
right hand sense
with respect to the direction of radiation.
An antenna emits polarized radio waves, with the polarization determined by the direction of the metal antenna elements. For example, a
dipole antenna
consists of two collinear metal rods. If the rods are horizontal it radiates horizontally polarized radio waves, while if the rods are vertical it radiates vertically polarized waves. An antenna receiving the radio waves must have the same polarization as the transmitting antenna, or it will suffer a severe loss of reception. Many natural sources of radio waves, such as the sun, stars and
blackbody radiation
from warm objects, emit unpolarized waves, consisting of incoherent short wave trains in an equal mixture of polarization states.
The polarization of radio waves is determined by a
quantum mechanical
property of the
photons
called their
spin
. A photon can have one of two possible values of spin; it can spin in a
right hand
sense about its direction of motion, or in a left hand sense. Right circularly polarized radio waves consist of photons spinning in a right hand sense. Left circularly polarized radio waves consist of photons spinning in a left hand sense. Plane polarized radio waves consist of photons in a
quantum superposition
of right and left hand spin states. The electric field consists of a superposition of right and left rotating fields, resulting in a plane oscillation.
Propagation characteristics
[
edit
]
Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable
propagation
properties, stemming from their large
wavelength
.
[10]
Radio waves have the ability to pass through the atmosphere in any weather, foliage, and most building materials, and by
diffraction
longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength.
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:
[11]
- 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
,
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.
- Indirect propagation
: Radio waves can reach points beyond the line-of-sight by
diffraction
and
reflection
.
[11]
Diffraction causes radio waves to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. Radio waves also partially 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 reception problems.
- 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 makes it possible for 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. VLF and ELF radio waves can also penetrate water to hundreds of meters depth, so they are used to
communicate with submerged submarines
.
- 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.
At
microwave
frequencies, atmospheric gases begin absorbing radio waves, so the range of practical radio communication systems decreases with increasing frequency. Below about 20 GHz atmospheric attenuation is mainly due to water vapor. Above 20 GHz, in the
millimeter wave
band, other atmospheric gases begin to absorb the waves, limiting practical transmission distances to a kilometer or less. Above 300 GHz, in the
terahertz band
, virtually all the power is absorbed within a few meters, so the atmosphere is effectively opaque.
[12]
[13]
Radio communication
[
edit
]
In
radio communication
systems, information is transported 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
.
[14]
The information, called the
modulation 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 wave
because it creates the radio waves that "carry" the information through the air. The information signal is used to
modulate
the carrier, altering some aspect of it, encoding 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.
[14]
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 one or more
tuned circuits
which act like a
resonator
, similarly to a
tuning fork
.
[14]
The tuned circuit 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
[
edit
]
Radio waves are
non-ionizing 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. 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 therapy
and to kill cancer cells.
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).
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 cause
cataracts
.
[15]
[16]
[17]
[18]
[19]
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. Radiofrequency electromagnetic fields have been classified by the
International Agency for Research on Cancer
(IARC) as having "limited evidence" for its effects on humans and animals.
[20]
[21]
There is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones.
[22]
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.
[23]
Measurement
[
edit
]
Since radio frequency radiation has both an electric and a magnetic component, it is often convenient to express intensity of radiation field in terms of units specific to each component. The unit
volts per meter
(V/m) is used for the electric component, and the unit
amperes per meter
(A/m) is used for the magnetic component. One can speak of an
electromagnetic field
, and these units are used to provide information about the levels of electric and magnetic
field strength
at a measurement location.
Another commonly used unit for characterizing an RF electromagnetic field is
power density
. Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the
far field
zone of the radiation pattern.
[24]
In closer proximity to the transmitter, i.e., in the "near field" zone, the physical relationships between the electric and magnetic components of the field can be complex, and it is best to use the field strength units discussed above. Power density is measured in terms of power per unit area, for example, milliwatts per square centimeter (mW/cm
2
). When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in the far field zone.
See also
[
edit
]
References
[
edit
]
- ^
Ellingson, Steven W. (2016).
Radio Systems Engineering
. Cambridge University Press. pp. 16?17.
ISBN
978-1316785164
.
- ^
"Ch. 1: Terminology and technical characteristics - Terms and definitions".
Radio Regulations
(PDF)
. Geneva, CH:
ITU
. 2016. p. 7.
ISBN
9789261191214
.
Archived
(PDF)
from the original on 2017-08-29.
- ^
Harman, Peter Michael (1998).
The natural philosophy of James Clerk Maxwell
. Cambridge, UK: Cambridge University Press. p. 6.
ISBN
0-521-00585-X
.
- ^
Edwards, Stephen A.
"Heinrich Hertz and electromagnetic radiation"
. American Association for the Advancement of Science
. Retrieved
13 April
2021
.
- ^
a
b
c
Gosling, William (1998).
Radio Antennas and Propagation
(PDF)
. Newnes. pp. 2, 12.
ISBN
0750637412
.
- ^
Shore, Bruce W. (2020).
Our Changing Views of Photons: A Tutorial Memoir
. Oxford University Press. p. 54.
ISBN
9780192607645
.
- ^
"Electromagnetic Frequency, Wavelength and Energy
Ultra
Calculator"
.
1728.org
. 1728 Software Systems
. Retrieved
15 Jan
2018
.
- ^
"How Radio Waves Are Produced"
.
NRAO
. Archived from
the original
on 28 March 2014
. Retrieved
15 Jan
2018
.
- ^
"Ch. 1: Terminology and technical characteristics - Terms and definitions".
ITU Radio Regulations
(PDF)
. Geneva, CH:
International Telecommunication Union
. 2016.
ISBN
9789261191214
.
Archived
(PDF)
from the original on 2017-08-29.
- ^
Ellingson, Steven W. (2016).
Radio Systems Engineering
. Cambridge University Press. pp. 16?17.
ISBN
978-1316785164
.
- ^
a
b
Seybold, John S. (2005).
"1.2 Modes of Propagation"
.
Introduction to RF Propagation
. John Wiley and Sons. pp. 3?10.
ISBN
0471743682
.
- ^
Coutaz, Jean-Louis; Garet, Frederic; Wallace, Vincent P. (2018).
Principles of Terahertz Time-Domain Spectroscopy: An Introductory Textbook
. CRC Press. p. 18.
ISBN
9781351356367
.
- ^
Siegel, Peter (2002).
"Studying the Energy of the Universe"
.
Education materials
. NASA website. Archived from
the original
on 20 June 2021
. Retrieved
19 May
2021
.
- ^
a
b
c
Brain, M. (7 Dec 2000).
"How Radio Works"
.
HowStuffWorks.com
. Retrieved
11 Sep
2009
.
- ^
Kitchen, Ronald (2001).
RF and Microwave Radiation Safety Handbook
(2nd ed.). Newnes. pp.
64
?65.
ISBN
0750643552
.
- ^
van der Vorst, Andre; Rosen, Arye; Kotsuka, Youji (2006).
RF/Microwave Interaction with Biological Tissues
. John Wiley & Sons. pp. 121?122.
ISBN
0471752045
.
- ^
Graf, Rudolf F.; Sheets, William (2001).
Build Your Own Low-power Transmitters: Projects for the Electronics Experimenter
. Newnes. p. 234.
ISBN
0750672447
.
- ^
Elder, Joe Allen; Cahill, Daniel F. (1984).
"Biological Effects of RF Radiation"
.
Biological Effects of Radiofrequency Radiation
.
US EPA
. pp. 5.116?5.119.
- ^
Hitchcock, R. Timothy; Patterson, Robert M. (1995).
Radio-Frequency and ELF Electromagnetic Energies: A handbook for health professionals
. Industrial Health and Safety Series. John Wiley & Sons. pp. 177?179.
ISBN
9780471284543
.
- ^
"IARC Classifies Radiofrequency Electromagnetic Fields as Possibly Carcinogenic to Humans"
(PDF)
.
www.iarc.fr
(Press release).
WHO
. 31 May 2011.
Archived
(PDF)
from the original on 2018-12-12
. Retrieved
9 Jan
2019
.
- ^
"Agents Classified by the IARC Monographs"
.
monographs.iarc.fr
. Volumes 1?123.
IARC
. 9 Nov 2018
. Retrieved
9 Jan
2019
.
- ^
Baan, R.; Grosse, Y.; Lauby-Secretan, B.; El Ghissassi, F. (2014).
"Radiofrequency Electromagnetic Fields: Evaluation of cancer hazards"
(PDF)
.
monographs.iarc.fr
(conference poster).
IARC
.
Archived
(PDF)
from the original on 2018-12-10
. Retrieved
9 Jan
2019
.
- ^
Kimmel, William D.; Gerke, Daryl (2018).
Electromagnetic Compatibility in Medical Equipment: A Guide for Designers and Installers
. Routledge. p. 6.67.
ISBN
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.
- ^
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. Science and Technology Department.
NAB
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ISBN
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.
Archived
from the original on 1 May 2018.
External links
[
edit
]
Look up
radio wave
in Wiktionary, the free dictionary.
|
---|
ELF
3 Hz/100 Mm
30 Hz/10 Mm
|
SLF
30 Hz/10 Mm
300 Hz/1 Mm
|
ULF
300 Hz/1 Mm
3 kHz/100 km
|
VLF
3 kHz/100 km
30 kHz/10 km
|
LF
30 kHz/10 km
300 kHz/1 km
|
MF
300 kHz/1 km
3 MHz/100 m
|
HF
3 MHz/100 m
30 MHz/10 m
|
VHF
30 MHz/10 m
300 MHz/1 m
|
UHF
300 MHz/1 m
3 GHz/100 mm
|
SHF
3 GHz/100 mm
30 GHz/10 mm
|
EHF
30 GHz/10 mm
300 GHz/1 mm
|
THF
300 GHz/1 mm
3 THz/0.1 mm
|
|
|
|
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| |
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and health
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