Radio waves

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Radio waves are electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum. A common use is to transport information through the atmosphere or outer space without wires. Radio waves are distinguished from other kinds of electromagnetic waves by their wavelength, a relatively long wavelength in the electromagnetic spectrum.

Diagram of the electric (E) and magnetic (H) fields of Radio Waves emanating from a radio transmitting antenna (small dark vertical line in the center). Radio waves are usually produced by electric current alternating at radio frequency flowing in a special purpose conductor, called an antenna. Antenna dimensions must generally be comparable to wavelength to work efficiently. Very long waves are not practical because of the enormous antennas needed to produce them, although they are sometimes produced by lightning. Radio waves are also produced by cosmic phenomena in deep space. Actually, any kind of reciprocating motion of electric charges or magnets can produce radio waves if it is fast enough. Although very impractical, even a person waving a charged stick very fast can produce faint radio waves.

[edit] Propagation

Propagation is a term that describes the travel of electromagnetic waves, there being three main modes of propagation. The first is a straight line travel: the manner that radio waves travel through deep space (ignoring the slight deviations caused by gravity under the theory of relativity). A second way is skip, which is bouncing between the surface of the earth and the ionosphere. Frequencies between 3 MHz and 30 MHz are most reliable for this kind of propagation, called High Frequency (see image at right). The third way is to hug the surface of the earth as it curves around. Radio waves of very low frequency most often travel this way.

Radio signals can also enter two ionospheric layers of differing electron densities and duct between them. The image at the right illustrates this. Two radio signals of differing elevation angles are broadcast into the ionosphere, where they split into ordinary (red) and extraordinary (green) components. In this example, the ordinary component began ducting between the E and F ionospheric regions.

Although this mode of radio wave propagation is less common than the skip mode, it is nonetheless an important mode because it permits radio signals to travel significant distances with little attenuation.

[edit] Discovery and utilization

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.

Main article: invention of radio

Radio waves were first predicted by mathematical work done in 1865 by James Clerk Maxwell. Maxwell noticed wave-like properties of light and similarities in electrical and magnetic observations and proposed equations that described light waves and radio waves as waves of electromagnetism that travel in space. In 1887 Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory. Many inventions followed, making practical use of radio waves to transfer information through space.

Nikola Tesla and Guglielmo Marconi are credited with inventing systems to allow radio waves to be used for communication.

[edit] Radio portion of the electromagnetic spectrum

Radio waves are divided up into bands by frequency (and corresponding wavelength) as shown in the radio frequency spectrum table below.

[edit] Notes

• Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.
• The ELF, SLF, ULF, and VLF bands overlap the AF (audio frequency) spectrum, which is approximately 20–20,000 Hz. However, sounds are transmitted by atmospheric compression and expansion, and not by electromagnetic energy.
• The SHF and EHF bands are sometimes not considered to be a part of the radio spectrum, forming their own microwave spectrum.

[edit] Named frequency bands

[edit] General

Broadcast Frequencies:

• Longwave AM Radio = 148.5 - 283.5 kHz (LF)
• Mediumwave AM Radio = 530 kHz - 1710 kHz (MF)
• TV Band I (Channels 2 - 6) = 54 MHz - 88 MHz (VHF)
• FM Radio Band II = 88 MHz - 108 MHz (VHF)
• TV Band III (Channels 7 - 13) = 174 MHz - 216 MHz (VHF)
• TV Bands IV & V (Channels 14 - 69) = 470 MHz - 806 MHz (UHF) [1]

For more information see the NTIA frequency allocation chart: http://www.ntia.doc.gov/osmhome/allochrt.html

[edit] Amateur radio frequencies

The range of allowed frequencies vary between countries. These are just some of the more common bands, often collectively termed shortwave. The article amateur radio contains another list.

[edit] IEEE US

[edit] EU, NATO, US ECM frequency designations

[edit] Waveguide frequency bands

[edit] References

• ITU-R Recommendation V.431: Nomenclature of the frequency and wavelength bands used in telecommunications. International Telecommunication Union, Geneva.

• IEEE Standard 521-2002: Standard Letter Designations for Radar-Frequency Bands

• AFR 55-44/AR 105-86/OPNAVINST 3430.9A/MCO 3430.1, 27 October 1964 superseded by AFR 55-44/AR 105-86/OPNAVINST 3430.1A/MCO 3430.1A, 6 December 1978: Performing Electronic Countermeasures in the United States and Canada, Attachment 1,ECM Frequency Authorizations.

[edit] See also

Look up Radio waves in
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Radio Portal

• Radio propagation
• Frequency allocation
• Radio astronomy
• Radio electronics

[edit] References

• PBS, Tesla - Master of Lightning - http://www.pbs.org/tesla/ll/ll_whoradio.html
• James Clerk Maxwell, "A Dynamical Theory of the Electromagnetic Field", Philosophical Transactions of the Royal Society of London 155, 459-512 (1865).
• Heinrich Hertz, "Electric waves: being researches on the propagation of electric action with finite velocity through space". Cornell University Library Historical Monographs Collection. {Reprinted by} Cornell University Library Digital Collections