Radio waves

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.

Contents 1 Discovery and utilization 2 Propagation 3 Radio communication 4 In medicine 5 See also 6 References

Discovery and utilization

Radio waves were first predicted by mathematical work done in 1865 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 practical the use of radio waves to transfer information through space.

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.

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).^[3] 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.^[4]

In medicine

Radio frequency (RF) energy has been used in medical treatments for over 75 years^[5] generally for minimally invasive surgeries and coagulation, including the treatment of sleep apnea.^[6] Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body.

See also

• Radio astronomy

References

1. ^ Harman, Peter Michael (1998). The natural philosophy of James Clerk Maxwell. Cambridge, England: Cambridge University Press. p. 6. ISBN 0-521-00585-X. 
2. ^ "Heinrich Hertz: The Discovery of Radio Waves". Juliantrubin.com. http://www.juliantrubin.com/bigten/hertzexperiment.html. Retrieved 2011-11-08. 
3. ^ Brain, Marshall (2000-12-07). "How Radio Works". HowStuffWorks.com. http://electronics.howstuffworks.com/radio8.htm. Retrieved 2009-09-11. 
4. ^ Brain, Marshall (2000-12-08). "How Oscillators Work". HowStuffWorks.com. http://electronics.howstuffworks.com/oscillator3.htm. Retrieved 2009-09-11. 
5. ^ Ruey J. Sung and Michael R. Lauer (2000). Fundamental approaches to the management of cardiac arrhythmias. Springer. p. 153. ISBN 9780792365594. http://books.google.com/?id=S1fWhl2c5zIC&pg=PA153&dq=rf+coagulation+75-years. 
6. ^ Melvin A. Shiffman, Sid J. Mirrafati, Samuel M. Lam and Chelso G. Cueteaux (2007). Simplified Facial Rejuvenation. Springer. p. 157. ISBN 9783540710967. http://books.google.com/?id=w1fQK21WK28C&pg=RA1-PA157&dq=rf+coagulation+sleep-apnea. 

• 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" (1893). Cornell University Library Historical Monographs Collection. Reprinted by Cornell University Library Digital Collections.
• Karl Rawer: "Wave Propagation in the Ionosphere". Kluwer, Dordrecht 1993. ISBN 0-7923-0775-5

Radio spectrum ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm

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