- Radio waves
Radio waves are electromagnetic waves occurring on the
radio frequencyportion of the electromagnetic spectrum. A common use is to transport informationthrough the atmosphereor outer spacewithout wires. Radio waves are distinguished from other kinds of electromagnetic waves by their wavelength, a relatively long wavelength in the electromagnetic spectrum.
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 in which radio waves travel through deep space (ignoring the slight deviations caused by gravity under the
theory of relativity). A second way is skip, which involves such waves 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
Discovery and utilization
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 Hertzdemonstrated 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, Jagadish Chandra Boseand Guglielmo Marconiare credited with inventing systems to allow radio waves to be used for communication.
Radio portion of the electromagnetic spectrum
Radio waves are divided up into bands by frequency (and corresponding
wavelength) as shown in the radio frequency spectrumtable below. Waveguidefrequency bands
*ITU-R Recommendation V.431: [http://www.itu.int/rec/R-REC-V.431/en 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.
PBS, Tesla - [http://www.pbs.org/tesla/ll/ll_whoradio.html Who Invented Radio?]
James Clerk Maxwell, " A Dynamical Theory of the Electromagnetic Field", "Philosophical Transactions of the Royal Society of London" 155, 459-512 (1865).
Heinrich Hertz: [http://historical.library.cornell.edu/cgi-bin/cul.cdl/docviewer?did=cdl334&view=50&frames=0&seq=5 "Electric waves; being researches on the propagation of electric action with finite velocity through space"] (1893). Cornell University Library Historical Monographs Collection. [http://www.amazon.com/dp/1429740361/ Reprinted by] Cornell University Library Digital Collections
Wikimedia Foundation. 2010.
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