Animation of ahalf-wave dipoleantenna radiating radio waves, showing theelectric field lines. The antenna in the center is two vertical metal rods connected to aradio transmitter (not shown). The transmitter applies analternating electric current to the rods, which charges them alternatelypositive (+) andnegative (−). Loops of electric field leave the antenna and travel away at thespeed of light; these are the radio waves. In this animation the action is shown slowed down tremendously.
Radio waves (formerly calledHertzian waves) are a type ofelectromagnetic radiation with the lowestfrequencies and the longestwavelengths in theelectromagnetic spectrum, typically with frequencies below 300gigahertz (GHz) and wavelengths greater than 1 millimeter (3⁄64 inch), about the diameter of a grain of rice. Radio waves with frequencies above about 1 GHz and wavelengths shorter than 30 centimeters are calledmicrowaves.[1] Like all electromagnetic waves, radio waves in vacuum travel at thespeed of light, and in the Earth's atmosphere at a slightly lower speed. Radio waves are generated bycharged particles undergoingacceleration, such as time-varyingelectric currents.[2] Naturally occurring radio waves are emitted bylightning andastronomical objects, and are part of theblackbody radiation emitted by all warm objects.[3]
Radio waves are generated artificially by an electronic device called atransmitter, which is connected to anantenna, which radiates the waves. They are received by another antenna connected to aradio receiver, which processes the received signal. Radio waves are very commonly used in modern technology for fixed and mobileradio communication,broadcasting,radar andradio 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 candiffract around obstacles like mountains and follow the contour of the Earth (ground waves), shorter waves can reflect off theionosphere and return to Earth beyond the horizon (skywaves), while much shorter wavelengths bend or diffract very little and travel on aline of sight, so their propagation distances are limited to the visual horizon.
To preventinterference between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called theInternational Telecommunication Union (ITU), which defines radio waves as "electromagnetic waves offrequencies arbitrarily lower than3000 GHz, propagated in space without artificial guide".[4] Theradio 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 calledmicrowaves.
Diagram of theelectric fields (E) andmagnetic fields (H) of radio waves emitted by amonopole radio transmittingantenna (small dark vertical line in the center). The E and H fields are perpendicular, as implied by the phase diagram in the lower right.
Radio waves were first predicted by the theory ofelectromagnetism that was proposed in 1867 by Scottish mathematical physicistJames Clerk Maxwell.[5] His mathematical theory, now calledMaxwell's equations, predicted that a coupledelectric andmagnetic field could travel through space as an "electromagnetic wave". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicistHeinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating electromagnetic waves lower in frequency than light, radio waves, in his laboratory,[6] showing that they exhibited the same wave properties as light:standing waves,refraction,diffraction, andpolarization. Italian inventorGuglielmo Marconi developed the first practical radio transmitters and receivers around 1894–1895. He received the 1909Nobel 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.
Animated diagram of ahalf-wave dipole antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiverR. Theelectric field (E, green arrows) of the incoming wave results in oscillation of theelectrons in the rods, charging the ends alternately positive(+) and negative(−). Since the length of the antenna is one half thewavelength of the wave, the oscillating field inducesstanding 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 resistanceR).
Radio waves are produced artificially by time-varyingelectric currents, consisting ofelectrons flowing back and forth in a specially shaped metal conductor called anantenna. An electronic device called aradio 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 aradio 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.
Fromquantum mechanics, like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of unchargedelementary particles calledphotons.[7] 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 acoherent emitter of photons, like alaser, so the radio photons are allin phase.[8][7] However, fromPlanck's relation, the energy of individual radio photons is extremely small,[7] from 10−22 to 10−30joules. So the antenna of even a very low power transmitter emits an enormous number of photons every second. Therefore, except for certainmolecular electron transition processes such as atoms in amaser emitting microwave photons, radio wave emission and absorption is usually regarded as a continuousclassical process, governed byMaxwell's equations.
Radio waves in vacuum travel at thespeed of light.[9][10] When passing through a material medium, they are slowed depending on the medium'spermeability andpermittivity. Air is tenuous enough that in the Earth's atmosphere radio waves travel at very nearly the speed of light.
Thewavelength is the distance from one peak (crest) of the wave's electric field to the next, and is inversely proportional to thefrequency of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is
where
Equivalently,, the distance that a radio wave travels in 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).
Like other electromagnetic waves, a radio wave has a property calledpolarization, which is defined as the direction of the wave's oscillatingelectric field perpendicular to the direction of motion. A plane-polarized radio wave has an electric field that oscillates in a plane perpendicular to the direction of motion. In ahorizontally polarized radio wave the electric field oscillates in a horizontal direction. In avertically polarized wave the electric field oscillates in a vertical direction. In acircularly polarized wave the electric field at any point rotates about the direction of travel, once per cycle. Aright circularly polarized wave rotates in a right-hand sense about the direction of travel, while aleft circularly polarized wave rotates in the opposite sense.[11]: p.21 The wave'smagnetic field is perpendicular to the electric field, and the electric and magnetic field are oriented in aright-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, adipole 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 andblackbody 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 aquantum mechanical property of thephotons called theirspin. 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 aquantum 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.
Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirablepropagation properties, stemming from their largewavelength.[12] Radio waves have the ability to pass through the atmosphere in any weather, foliage, and through most building materials. Bydiffraction, 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 ofradio 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 experiencereflection,refraction,polarization,diffraction, andabsorption. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certainradio bands more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate:[13]
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 visualhorizon to about 64 km (40 mi). This is the method used bycell phones,FM,television broadcasting andradar. By usingdish antennas to transmit beams of microwaves, point-to-pointmicrowave relay links transmit telephone and television signals over long distances up to the visual horizon.Ground stations can communicate withsatellites and spacecraft billions of miles from Earth.
Indirect propagation: Radio waves can reach points beyond the line-of-sight bydiffraction andreflection.[13] 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 ascell phones,cordless phones,walkie-talkies, andwireless networks. A drawback of this mode ismultipath propagation, in which radio waves travel from the transmitting to the receiving antenna via multiple paths. The wavesinterfere, often causingfading and other reception problems.
Ground waves: At lower frequencies below 2 MHz, in themedium wave andlongwave bands, due to diffractionvertically polarized radio waves can bend over hills and mountains, and propagate beyond the horizon, traveling assurface 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. Ground waves are gradually absorbed by the Earth, so the power density of the waves decreases exponentially with distance from the transmitting antenna, limiting the range of reception. As the frequency drops, the losses decrease and the achievable range increases. Militaryvery low frequency (VLF) andextremely 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 deep, so they are used tocommunicate with submerged submarines.
Skywaves: Atmedium wave andshortwave wavelengths, radio waves reflect off conductive layers of charged particles (ions) in a part of the atmosphere called theionosphere. 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 militaryover-the-horizon (OTH) radar systems, by some automated systems, byradio amateurs, and by shortwave broadcasting stations to broadcast to other countries.
Atmicrowave 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 themillimeter wave band, other atmospheric gases begin to absorb the waves, limiting practical transmission distances to a kilometer or less. Above 300 GHz, in theterahertz band, virtually all the power is absorbed within a few meters, so the atmosphere is effectively opaque.[14][15]
Inradio 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 aradio transmitter.[16] The information, called themodulation signal, can be anaudio signal representing sound from amicrophone, avideo signal representing moving images from avideo camera, or adigital signal representing data from acomputer. In the transmitter, anelectronic oscillator generates analternating current oscillating at aradio frequency, called thecarrier wave because it creates the radio waves that "carry" the information through the air. The information signal is used tomodulate the carrier, altering some aspect of it, encoding the information on the carrier. The modulated carrier is amplified and applied to anantenna. The oscillating current pushes theelectrons in the antenna back and forth, creating oscillatingelectric andmagnetic 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.[16] This voltage is applied to theradio receiver, which extracts the information signal. The receiver first uses abandpass filter to separate the desired radio station's radio signal from all the other radio signals picked up by the antenna, thenamplifies the signal so it is stronger, then finally extracts the information-bearing modulation signal in ademodulator. The recovered signal is sent to aloudspeaker orearphone to produce sound, or a televisiondisplay screen to produce a visible image, or other devices. A digital data signal is applied to acomputer ormicroprocessor, 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 differentfrequency, measured inkilohertz (kHz),megahertz (MHz) orgigahertz (GHz). Thebandpass filter in the receiver consists of one or moretuned circuits which act like aresonator, similarly to atuning fork.[16] The tuned circuit has a naturalresonant 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.
Radio waves arenon-ionizing radiation, which means they do not have enough energy to separateelectrons fromatoms ormolecules,ionizing them, or breakchemical bonds, causing chemical reactions orDNA damage. The main effect of absorption of radio waves by materials is to heat them, similarly to theinfrared waves radiated by sources of heat such as aspace heater or wood fire. The oscillating electric field of the wave causespolar molecules to vibrate back and forth, increasing the temperature; this is how amicrowave oven cooks food. Radio waves have been applied to the body for 100 years in the medical therapy ofdiathermy for deep heating of body tissue, to promote increased blood flow and healing. More recently they have been used to create higher temperatures inhyperthermia 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'sresistivity andpermittivity; it is given by a parameter called theskin 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 approximately2.5 to 3.8 cm.
Radio waves symbol
Looking into a source of radio waves at close range, such as thewaveguide 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 causecataracts.[17][18][19][20][21]
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 theInternational Agency for Research on Cancer (IARC) as having "limited evidence" for its effects on humans and animals.[22][23] There is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones.[24]
Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called aFaraday 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 about1⁄20 ofwavelength of the waves.[25]
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 unitvolt per meter (V/m) is used for the electric component, and the unitampere per meter (A/m) is used for the magnetic component. One can speak of anelectromagnetic field, and these units are used to provide information about the levels of electric and magneticfield strength at a measurement location.
Another commonly used unit for characterizing an RF electromagnetic field ispower 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 thefar field zone of the radiation pattern.[26] 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, with the unit milliwatt 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.
^"Ch. 1: Terminology and technical characteristics - Terms and definitions".Radio Regulations(PDF). Geneva, CH:ITU. 2016. p. 7.ISBN9789261191214.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.ISBN0-521-00585-X.
^Elder, Joe Allen; Cahill, Daniel F. (1984)."Biological Effects of RF Radiation".Biological Effects of Radiofrequency Radiation.US EPA. pp. 5.116 –5.119.Archived from the original on 2024-09-22. Retrieved2019-08-16.
Rawer, Karl (1993).Wave Propagation in the Ionosphere. Developments in electromagnetic theory and applications series. Dordrecht: Kluwer Academic.ISBN9780792307754.OCLC26257685.
