Radio wave

Discovery and exploitation

Radio waves were first predicted by the theory of

Generation and reception

. 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

Planck's relation

${\displaystyle E=h\nu }$ the energy of individual radio photons is extremely small,

Radio waves in a vacuum travel at the speed of light ${\displaystyle c}$ .^[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 ${\displaystyle \lambda }$ is the distance from one peak (crest) of the wave's electric field to the next, and is inversely proportional to the frequency ${\displaystyle f}$ of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is

${\displaystyle \lambda ={\frac {\;c\;}{f}}~,}$

where

${\displaystyle c\approx 299.79\times 10^{6}{\text{ m/s}}~.}$

Equivalently, ${\displaystyle \;c\;}$ 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 

Polarization

Like other electromagnetic waves, a radio wave has a property called

An antenna emits polarized radio waves, with the polarization determined by the direction of the metal antenna elements. For example, a

The polarization of radio waves is determined by a

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Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable propagation properties, stemming from their large wavelength.^[9] 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

• satellites
  and spacecraft billions of miles from Earth.
  • Indirect propagation: Radio waves can reach points beyond the line-of-sight by
    interfere, often causing fading
    and other reception problems.
• 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
  .
• radio amateurs
  , and by shortwave broadcasting stations to broadcast to other countries.

At

terahertz band, virtually all the power is absorbed within a few meters, so the atmosphere is effectively opaque.^[11][12]

Radio communication

Main article: Radio

In

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.

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

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

Further information: Medical applications of radio frequency

Radio waves are

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

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.^{[14][15][16][17][18]} Nevertheless, 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.^[19][20] There is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones.^[21]

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.^[22]

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.^[23]

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.

See also

• Radio astronomy
• Television transmitter

References

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2. ISBN 9789261191214. Archived
   (PDF) from the original on 2017-08-29.
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4. ^ Edwards, Stephen A. "Heinrich Hertz and electromagnetic radiation". American Association for the Advancement of Science. Retrieved 13 April 2021.
5. ^
   ISBN 0750637412
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6. ISBN 9780192607645
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7. ^ "Electromagnetic Frequency, Wavelength and Energy Ultra Calculator". 1728.org. 1728 Software Systems. Retrieved 15 Jan 2018.
8. ^ "How Radio Waves Are Produced". NRAO. Archived from the original on 28 March 2014. Retrieved 15 Jan 2018.
9. ISBN 978-1316785164
   .
10. ^
    ISBN 0471743682
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11. ISBN 9781351356367
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12. ^ 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.
13. ^ ^{a b c} Brain, M. (7 Dec 2000). "How Radio Works". HowStuffWorks.com. Retrieved 11 Sep 2009.
14. ISBN 0750643552
    .
15. ISBN 0471752045
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16. ISBN 0750672447
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17. US EPA
    . pp. 5.116–5.119.
18. ISBN 9780471284543
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19. ^ "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.
20. ^ "Agents Classified by the IARC Monographs". monographs.iarc.fr. Volumes 1–123. IARC. 9 Nov 2018. Retrieved 9 Jan 2019.
21. ^ 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.
22. ISBN 9781351453370
    .
23. ISBN 9780893242367. Archived
    from the original on 1 May 2018.

• S2CID 186207827
  .
• ISBN 9781429740364
  .
• OCLC 26257685
  .

External links

Wikiquote has quotations related to Radio wave.

Look up radio wave in Wiktionary, the free dictionary.

• "Radio Waves". Science Mission Directorate. NASA.

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