TheInfoList

Radio waves are a type of
electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through ...
with
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adjac ...

s in the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without steps, across a Continuum (theory), ...
longer than
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from the n ...

radiation. It consist of the longest wavelength electromagnetic waves, typically with
frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

300 gigahertz (
GHz The hertz (symbol: Hz) is the derived unit of frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and an ...
) and below. At 300 GHz, the corresponding
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adjac ...

is 1 mm (shorter than a grain of rice); at 30 Hz the corresponding wavelength is 10,000 km (longer than the radius of the Earth). Like all electromagnetic waves, radio waves in a vacuum travel at the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant important in many areas of physics. Its exact value is defined as (approximately ). It is exact because, by international agreement, a Metre#Speed of light def ...
, and in the Earth's atmosphere at a close, but slightly lower speed. Radio waves are generated by
charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
s undergoing
acceleration In mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, more specifically the relationships among force, matter, and motion. Forces applied to objects result in Displacement ( ...
, such as time-varying
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving part ...
s. Naturally occurring radio waves are emitted by
lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions in the atmosphere or ground temporarily equalize themselves, causing the instantaneous release of as much as one gigajoule of ene ...

and
astronomical objects In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...
, and are part of the
blackbody radiation ) of black-body radiation scales inversely with the temperature of the black body; the locus of such colors, shown here in CIE 1931 ''x,y'' space, is known as the Planckian locus. Black-body radiation is the thermal radiation, thermal electrom ...
emitted by all warm objects. Radio waves are generated artificially by
transmitter In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplific ...

s, using
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
s. Radio waves are very widely used in modern technology for fixed and mobile
radio communication Radio is the technology of signaling and communicating using radio wave Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as hig ...
,
broadcasting Broadcasting is the distribution (business), distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications medium, but typically one using the electromagnetic spectrum (radio ...
,
radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Ear ...

and
systems,
communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a Transponder (satellite communications), transponder; it creates a communication channel between a source transmitter and a Radio ...
s, 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 wave A diving thumb In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and beh ...
s), shorter waves can reflect off the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere and parts of the mesosphere upright=0.5, Diagram showing the five primary layers of the Earth's atmosphere: exos ...
skywave off the ionosphere (red) during skywave propagation In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper a ...

s), while much shorter wavelengths bend or diffract very little and travel on a
line of sight Line of sight (adjectival form line-of-sight) may refer to: Common meanings * Sightline, an unobstructed line-of-sight between a subject and object * Line of sight (gaming), visibility on a gaming field, ''i.e.'' who can see what Arts and enterta ...
, so their propagation distances are limited to the visual horizon. To prevent
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extran ...
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 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of the United Nations United Nations Specialized Agencies are autonomous organizations working with the United Nations and each other through the ...

(ITU), which defines radio waves as "
electromagnetic wave Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

s of
frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

arbitrarily lower than 3,000
GHz The hertz (symbol: Hz) is the derived unit of frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and an ...
, propagated in space without artificial guide". The
radio spectrum The radio spectrum is the part of the electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of ...
is divided into a number of radio bands on the basis of frequency, allocated to different uses.

# Discovery and exploitation

Radio waves were first predicted by mathematical work done in 1867 by Scottish mathematical physicist
James Clerk Maxwell James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of interest. In classica ...

. His mathematical theory, now called
Maxwell's equations Maxwell's equations are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. The equations provide a mathematic ...
, predicted that a coupled
electric Electricity is the set of physics, physical Phenomenon, phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnet ...
and
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

could travel through space as an "
electromagnetic wave Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's Maxwell's equations, equations of electroma ...

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 wave In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Sp ...

s,
refraction In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior throu ...

,
diffraction Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. It is defined as the bending of waves around the corners of an obstacle or through an aperture In optics, an aperture is a hole or an openin ...
, and
polarization Polarization or polarisation may refer to: In the physical sciences *Polarization (waves), the ability of waves to oscillate in more than one direction, in particular polarization of light, responsible for example for the glare-reducing effect of ...
. Italian inventor
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italian inventor and electrical engineer, known for his pioneering work on long-distance radio transmission, development of Marconi's law, and a ...

Nobel Prize in physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
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

charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
s when they are accelerated. They are produced artificially by time-varying
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving part ...
s, consisting of
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...
s flowing back and forth in a specially-shaped metal conductor called an
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
. An electronic device called a
radio transmitter In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplific ...
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

. 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 Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quan ...
, like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged
elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, leptons, antiquarks, and an ...
s called ''
photon The photon (Greek: φῶς, phōs, light) is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light Light or visible light is electromagnetic radiation within the p ...

s''. 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 A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radia ...

, so the radio photons are all
in phase In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
. However, from Planck's relation $E = h\nu$ the energy of individual radio photons is extremely small, from 10−22 to 10−30
joule The joule ( ; symbol: J) is a SI derived unit, derived unit of energy in the International System of Units. It is equal to the energy transferred to (or work (physics), work done on) an object when a force of one Newton (unit), newton acts on tha ...

s. It is so small that, except for certain
molecular electron transition Molecular electronic transitions take place when electron The electron is a subatomic particle, symbol or , whose electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an ele ...
processes such as atoms in a
maser (see description below) A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherence (physics), coherent electromagnetic waves In physics Physics (from grc, φυσικ ...

emitting microwave photons, radio wave emission and absorption is usually regarded as a continuous
classical Classical may refer to: European antiquity *Classical antiquity, a period of history from roughly the 7th or 8th century B.C.E. to the 5th century C.E. centered on the Mediterranean Sea *Classical architecture, architecture derived from Greek and ...
process, governed by
Maxwell's equations Maxwell's equations are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. The equations provide a mathematic ...
.

# Properties

Radio waves in a vacuum travel at the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant important in many areas of physics. Its exact value is defined as (approximately ). It is exact because, by international agreement, a Metre#Speed of light def ...
$c$ . When passing through a material medium, they are slowed depending on the medium's permeability and
permittivity In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is car ...
. Air is thin enough that in the Earth's atmosphere radio waves travel very close to the speed of light. The
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adjac ...

$\lambda$ is the distance from one peak (crest) of the wave's electric field to the next, and is inversely proportional to the
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

$f$ of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is :$\lambda = \frac~,$ where :$c \approx 299.79 \times 10^6 \text~.$ Equivalently, $\;c\;$ the distance a radio wave travels in a vacuum, in one second, is , which is the wavelength of a 1
hertz The hertz (symbol: Hz) is the unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatric ...
megahertz The hertz (symbol: Hz) is the SI derived unit, derived unit of frequency in the International System of Units (SI) and is defined as one cycle (geometry), cycle per second. It is named after Heinrich Hertz, Heinrich Rudolf Hertz, the first per ...
AM band Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM broadcasting, AM radio broadcasting. The spectrum provides about 120 channels with limited sound quality. During daytime, only local stations can be received. ...
) has a wavelength of .

## Polarization

Like other electromagnetic waves, a radio wave has a property called
polarization Polarization or polarisation may refer to: In the physical sciences *Polarization (waves), the ability of waves to oscillate in more than one direction, in particular polarization of light, responsible for example for the glare-reducing effect of ...
, which is defined as the direction of the wave's oscillating
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' ' ...
perpendicular to the direction of motion. A plane polarized radio wave has an electric field which oscillates in a plane along the direction of motion. In a horizontally polarized radio wave the electric field oscillates in a horizontal direction. In a 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 In human biology, handedness is the better, faster, or more precise performance or individual preference for use of a hand, known as the dominant hand. The incapable, less capable or less preferred hand is called the non-dominant hand. Right-han ...

sense about the direction of travel, while a left circularly polarized wave rotates in the opposite sense. The wave's
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

is perpendicular to the electric field, and the electric and magnetic field are oriented in a 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 In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna (radio), antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary el ...
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 ) of black-body radiation scales inversely with the temperature of the black body; the locus of such colors, shown here in CIE 1931 ''x,y'' space, is known as the Planckian locus. Black-body radiation is the thermal radiation, thermal electrom ...
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 Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quant ...
property of the
photon The photon (Greek: φῶς, phōs, light) is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light Light or visible light is electromagnetic radiation within the p ...

s called their
spin Spin or spinning may refer to: Businesses * SPIN (cable system) or South Pacific Island Network * Spin (company), an American scooter-sharing system * SPiN, a chain of table tennis lounges Computing * SPIN model checker, Gerard Holzmann's tool fo ...
. A photon can have one of two possible values of spin; it can spin in a
right hand In human biology, handedness is the better, faster, or more precise performance or individual preference for use of a hand, known as the dominant hand. The incapable, less capable or less preferred hand is called the non-dominant hand. Right-han ...

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 Quantum superposition is a fundamental principle of quantum mechanics Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and sub ...
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

Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable propagation properties, stemming from their large
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adjac ...

. Radio waves have the ability to pass through the atmosphere in any weather, foliage, and most building materials, and by
diffraction Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. It is defined as the bending of waves around the corners of an obstacle or through an aperture In optics, an aperture is a hole or an openin ...
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 Radio propagation is the behavior of radio wave Radio waves are a type of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'n ...
, 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
reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal r ...
,
refraction In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior throu ...

,
polarization Polarization or polarisation may refer to: In the physical sciences *Polarization (waves), the ability of waves to oscillate in more than one direction, in particular polarization of light, responsible for example for the glare-reducing effect of ...
,
diffraction Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. It is defined as the bending of waves around the corners of an obstacle or through an aperture In optics, an aperture is a hole or an openin ...
, and
absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...
. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain
radio band The radio spectrum is the part of the electromagnetic spectrum with Frequency, frequencies from 3 hertz, Hz to 300 Hertz, GHz. Electromagnetic waves in this frequency range, called ''radio waves'', are widely used in modern technology, ...
s more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate: * ''
Line of sight Line of sight (adjectival form line-of-sight) may refer to: Common meanings * Sightline, an unobstructed line-of-sight between a subject and object * Line of sight (gaming), visibility on a gaming field, ''i.e.'' who can see what Arts and enterta ...
:'' 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 The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is o ...

to about 64 km (40 mi). This is the method used by
cell phone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunications device that permits two or more use ...

s, FM,
television broadcasting A television network or broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid-1980s, te ...

and
radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Ear ...

. By using
dish antenna antenna at Erdfunkstelle Raisting, the biggest facility for satellite communication in the world, in Raisting, Bavaria Bavaria (; German language, German and Bavarian language, Bavarian: ''Bayern'' ), officially the Free State of Bavaria (German ...
s to transmit beams of microwaves, point-to-point
microwave relay Microwave transmission is the transmission of information by microwave radio waves. Although an experimental microwave telecommunication link across the English Channel was demonstrated in 1931, the development of radar in World War II ...
links transmit telephone and television signals over long distances up to the visual horizon. Ground stations can communicate with
satellites alt=, A full-size model of the Earth observation satellite ERS 2 ">ERS_2.html" ;"title="Earth observation satellite ERS 2">Earth observation satellite ERS 2 In the context of spaceflight, a satellite is an object that has been intentionally ...
and spacecraft billions of miles from Earth. ** ''Indirect propagation'': Radio waves can reach points beyond the line-of-sight by ''
diffraction Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. It is defined as the bending of waves around the corners of an obstacle or through an aperture In optics, an aperture is a hole or an openin ...
'' 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 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 phone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunications device that permits two or more use ...

s,
cordless phone The term cordless is generally used to refer to electrical or electronic devices that are powered by a battery (electricity), battery or battery pack and can operate without a power cord or cable attached to an electrical outlet to provide mains po ...
s,
walkie-talkie A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer Alfred J. Gross, He ...
s, and
wireless network A wireless network is a computer network that uses wireless data connections between network nodes. Wireless networking is a method by which homes, telecommunications networks and business installations avoid the costly process of introducing ca ...

s. A drawback of this mode is ''
multipath propagation In radio communication, multipath is the propagation phenomenon that results in radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30& ...
'', in which radio waves travel from the transmitting to the receiving antenna via multiple paths. The waves interfere, often causing
fading In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...

and other reception problems. * ''
Ground wave A diving thumb The thumb is the first digit of the hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The ter ...
s:'' At lower frequencies below 2 MHz, in the
medium wave Medium wave (MW) is the part of the medium frequency Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300  kilohertz (kHz) to 3  megahertz (MHz). Part of this band is the medium wave (M ...
and
longwave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the ea ...
bands, due to diffraction radio waves can bend over hills and mountains, and propagate beyond the horizon, traveling as
surface wave A diving thumb The thumb is the first digit of the hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The ter ...

s 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 Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30  kHz, corresponding to wavelength In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), kno ...
(VLF) and
extremely low frequency Extremely low frequency (ELF) is the ITU designation for electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is th ...
(ELF) communication systems can communicate over most of the Earth, and with submarines hundreds of meters underwater. * ''
Skywave off the ionosphere (red) during skywave propagation In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper a ...

s:'' At
medium wave Medium wave (MW) is the part of the medium frequency Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300  kilohertz (kHz) to 3  megahertz (MHz). Part of this band is the medium wave (M ...
and
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the High frequency, high frequency band (HF), which extends from 3 to 30 MHz (100 ...

wavelengths, radio waves reflect off conductive layers of charged particles (
ions An ion () is an atom or molecule with a net electric charge, electrical charge. The charge of an electron is considered negative by convention and this charge is equal and opposite to charge of a proton, which is considered positive by convent ...

) in a part of the atmosphere called the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere and parts of the mesosphere upright=0.5, Diagram showing the five primary layers of the Earth's atmosphere: exos ...
. 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 amateur Radio is the technology of signaling and communicating using radio wave Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as hig ...
s, and by shortwave broadcasting stations to broadcast to other countries. At
microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...

frequencies, atmospheric gases begin absorbing radio waves, so the range of practical radio communication systems decreases with the augmentation of the frequency. Below about 20 GHz atmospheric attenuation is mainly due to water vapor. Above 20 GHz, in the
millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and ...
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 Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic wave Electromagnetism is a branch of physic ...
, virtually all the power is absorbed within a few meters, so the atmosphere is effectively opaque.

In
radio communication Radio is the technology of signaling and communicating using radio wave Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as hig ...
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 In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplific ...
. The information, called the , can be an
audio signal Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion ( ...
representing sound from a
microphone A microphone, colloquially called a mic or mike (), is a device – a transducer – that converts sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''ph ...

, a
video signal Video is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active de ...
representing moving images from a
video camera A video camera is a camera used for electronic motion picture acquisition (as opposed to a movie camera, which records images on filmstock, film), initially developed for the television industry but now common in other applications as well. Vide ...

, or a
digital signal A digital signal is a signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image processing, images ...
representing data from a
computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These programs enable compu ...

. In the transmitter, an
electronic oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillation, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillation, Oscillators convert direct current (DC) from a power suppl ...
generates an
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...
oscillating at a
radio frequency Radio frequency (RF) is the oscillation rate of an Alternating_current, alternating electric current or voltage or of a Magnetic_field, magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around ...
, called the ''
carrier wave Image:Modulated radio signal frequency spectrum.svg, upright=1.4, The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal amplitude or power. It consists of a ...
'' because it creates the radio waves that "carry" the information through the air. The information signal is used to the carrier, altering some aspect of it, "piggybacking" the information on the carrier. The modulated carrier is amplified and applied to an
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
. The oscillating current pushes the
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...
s in the antenna back and forth, creating oscillating
electric Electricity is the set of physics, physical Phenomenon, phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnet ...
and
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

s, 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

, which extracts the information signal. The receiver first uses a
bandpass filter File:Bandpass_Filter.svg, 300px, A medium-complexity example of a band-pass filter. A band-pass filter or bandpass filter (BPF) is a device that passes frequency, frequencies within a certain range and rejects (attenuates) frequencies outside t ...
to separate the desired radio station's radio signal from all the other radio signals picked up by the antenna, then the signal so it is stronger, then finally extracts the information-bearing modulation signal in a
demodulator Demodulation is extracting the original information-bearing signal from a carrier wave upright=1.4, The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal ...
. The recovered signal is sent to a
loudspeaker A loudspeaker (or ''speaker driver'', or most frequently just ''speaker'') is an electroacoustic transducer, that is, a device that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referre ...

or
earphone Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound In physics Physics (from grc ...

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 A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These programs enable compu ...

or
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circuitr ...

, 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 Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

, measured in
kilohertz The hertz (symbol: Hz) is the unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatric ...
(kHz),
megahertz The hertz (symbol: Hz) is the SI derived unit, derived unit of frequency in the International System of Units (SI) and is defined as one cycle (geometry), cycle per second. It is named after Heinrich Hertz, Heinrich Rudolf Hertz, the first per ...
(MHz) or
gigahertz The hertz (symbol: Hz) is the SI derived unit, derived unit of frequency in the International System of Units (SI) and is defined as one cycle (geometry), cycle per second. It is named after Heinrich Hertz, Heinrich Rudolf Hertz, the first per ...
(GHz). The
bandpass filter File:Bandpass_Filter.svg, 300px, A medium-complexity example of a band-pass filter. A band-pass filter or bandpass filter (BPF) is a device that passes frequency, frequencies within a certain range and rejects (attenuates) frequencies outside t ...
in the receiver consists of a
tuned circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can act ...
which acts like a
resonator A resonator is a device or system that exhibits resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of a Periodic function, periodically applied force (or a Fourier analysis, Fourier component ...
, similarly to a tuning fork. It has a natural
resonant frequency Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelen ...
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

non-ionizing radiation Non-ionizing (or non-ionising) radiation refers to any type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion an ...
'', which means they do not have enough energy to separate
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...
s from
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

s or
molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms held together by chemical bo ...

s, ionizing them, or break
chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
s, causing chemical reactions or
DNA damage DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolism, metabolic activities and environmental factors such as ra ...
. The main effect of absorption of radio waves by materials is to heat them, similarly to the
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from the n ...

waves radiated by sources of heat such as a
space heater A space heater is a device used to heat a single, small to medium sized area. This contrasts with central heating upHot water central heating unit, using wood as fuel A central heating system provides warmth to the number of spaces within a buil ...

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 In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is car ...
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adjac ...

of the waves.

# Measurement

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. 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/cm2). 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.