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WHAT MAKES RADIO WAVES? 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. PROPAGATION OF RADIO WAVES Propagation is a term that describes the travel of electromagnetic waves, there being three principle 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. 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. HISTORICAL DISCOVERY 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. Italian engineer Guglielmo Marconi is generally credited with inventing radio. In 1895 he transmitted a signal 2.4 kilometres in the grounds of his father's property. He patented the first 'wireless telegraphy' system in 1896. RADIO PORTION OF THE ELECTROMAGNETIC WAVE SPECTRUM Radio waves and are divided up into bands by frequency (and corresponding Wavelength ) as shown in the radio Frequency Spectrum table below. Notes
NAMED FREQUENCY BANDS General Broadcast Frequencies:
For more information see the NTIA frequency allocation chart: http://www.ntia.doc.gov/osmhome/allochrt.html 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. IEEE US EU, NATO, US ECM Frequency Designations REFERENCES
SEE ALSO REFERENCES
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