Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight.
Diagram of the
electric fields (E) and
magnetic fields (H) of radio waves emitted by a
monopole radio transmitting
antenna (small dark vertical line in the center). The E and H fields are perpendicular as implied by the phase diagram in the lower right.
[edit] Discovery and utilization
Radio waves were first predicted by mathematical work done in 1867 by James Clerk Maxwell.[1] Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. He then 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.[2] Many inventions followed, making the use of radio waves to transfer information through space.
[edit] Propagation
The study of electromagnetic phenomena such as reflection, refraction, polarization, diffraction and absorption is of critical importance in the study of how radio waves move in free space and over the surface of the Earth. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others.
[edit] Speed, wavelength and frequency
Radio waves travel at the speed of light in a vacuum.[3][4] If radio waves strike an electrically conductive object of any size, they are slowed according to that object's permeability and permativity.
The wavelength is the distance from one 'peak' of magnetic flux to the next, or the peak of one 'wave' to the next, and is inversely proportional to the frequency. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters which is the wavelength of a 1 Hertz radio signal. A 1 Megahertz Radio signal has a wavelength of 299 meters.
[edit] Radio communication
In order to receive radio signals, for instance from AM/FM radio stations, a radio antenna must be used. However, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune in to a particular frequency (or frequency range).[5] This is typically done via a resonator (in its simplest form, a circuit with a capacitor and an inductor). The resonator is configured to resonate at a particular frequency (or frequency band), thus amplifying sine waves at that radio frequency, while ignoring other sine waves. Usually, either the inductor or the capacitor of the resonator is adjustable, allowing the user to change the frequency at which it resonates.[6]
[edit] In medicine
Radio frequency (RF) energy has been used in medical treatments for over 75 years[7] generally for minimally invasive surgeries and coagulation, including the treatment of sleep apnea.[8] Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body.
[edit] See also
[edit] References
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ELF
3 Hz/100 Mm
30 Hz/10 Mm
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SLF
30 Hz/10 Mm
300 Hz/1 Mm
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ULF
300 Hz/1 Mm
3 kHz/100 km
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VLF
3 kHz/100 km
30 kHz/10 km
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LF
30 kHz/10 km
300 kHz/1 km
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MF
300 kHz/1 km
3 MHz/100 m
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HF
3 MHz/100 m
30 MHz/10 m
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VHF
30 MHz/10 m
300 MHz/1 m
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UHF
300 MHz/1 m
3 GHz/100 mm
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SHF
3 GHz/100 mm
30 GHz/10 mm
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EHF
30 GHz/10 mm
300 GHz/1 mm
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THF
300 GHz/1 mm
3 THz/0.1 mm
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