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.

Sources

File:RadioWaves.jpg
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). The E and H fields are perpendicular as implied by the phase diagram in the lower right.

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

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.

Discovery and utilization

File:Atmospheric electromagnetic transmittance or opacity.jpg
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.

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.

Band name Abbr ITU band Frequency
and
Wavelength in air
Example uses
< 3 Hz
> 100,000 km
Extremely low frequency ELF 1 3–30 Hz
100,000 km – 10,000 km
Communication with submarines
Super low frequency SLF 2 30–300 Hz
10,000 km – 1000 km
Communication with submarines
Ultra low frequency ULF 3 300–3000 Hz
1000 km – 100 km
Communication within mines
Very low frequency VLF 4 3–30 kHz
100 km – 10 km
Submarine communication, avalanche beacons, wireless heart rate monitors, geophysics
Low frequency LF 5 30–300 kHz
10 km – 1 km
Navigation, time signals, AM longwave broadcasting
Medium frequency MF 6 300–3000 kHz
1 km – 100 m
AM (Medium-wave) broadcasts
High frequency HF 7 3–30 MHz
100 m – 10 m
Shortwave broadcasts, amateur radio and over-the-horizon aviation communications
Very high frequency VHF 8 30–300 MHz
10 m – 1 m
FM, television broadcasts and line-of-sight ground-to-aircraft and aircraft-to-aircraft communications
Ultra high frequency UHF 9 300–3000 MHz
1 m – 100 mm
television broadcasts, microwave ovens, mobile phones, wireless LAN, Bluetooth, GPS and Two-Way Radios such as FRS and GMRS Radios
Super high frequency SHF 10 3–30 GHz
100 mm – 10 mm
microwave devices, wireless LAN, most modern Radars
Extremely high frequency EHF 11 30–300 GHz
10 mm – 1 mm
Radio astronomy, high-speed microwave radio relay
Above 300 GHz
< 1 mm

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.

Named frequency bands

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

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.

Band Frequency range
160 m 1.8 to 2.0 MHz
80 m 3.5 to 4.0 MHz
60 m 5.3 to 5.4 MHz
40 m 7 to 7.3 MHz
30 m 10.1 to 10.15 MHz
20 m 14 to 14.35 MHz
15 m 21 to 21.45 MHz
12 m 24.89 to 24.99 MHz
10 m 28.0 to 29.7 MHz
6 m 50 to 54 MHz
2 m 144 to 148 MHz
70 cm 430 to 440 MHz
33 cm 902 to 928 MHz
23 cm 1240 to 1300 MHz

IEEE US

Band Frequency range Origin of name
HF band 3 to 30 MHz High Frequency
VHF band 30 to 300 MHz Very High Frequency
UHF band 300 to 3000 MHz Ultra High Frequency

Frequencies from 216 to 450 MHz were sometimes called P-band: Previous, since early British Radar used this band but later switched to higher frequencies.

L band 1 to 2 GHz Long wave
S band 2 to 4 GHz Short wave
C band 4 to 8 GHz Compromise between S and X
X band 8 to 12 GHz Used in WW II for fire control, X for cross (as in crosshair)
Ku band 12 to 18 GHz Kurz-under
K band 18 to 26 GHz German Kurz (short)
Ka band 26 to 40 GHz Kurz-above
V band 40 to 75 GHz
W band 75 to 111 GHz W follows V in the alphabet

EU, NATO, US ECM frequency designations

Band Frequency range
A band 0 to 0.25 GHz
B band 0.25 to 0.5 GHz
C band 0.5 to 1.0 GHz
D band 1 to 2 GHz
E band 2 to 3 GHz
F band 3 to 4 GHz
G band 4 to 6 GHz
H band 6 to 8 GHz
I band 8 to 10 GHz
J band 10 to 20 GHz
K band 20 to 40 GHz
L band 40 to 60 GHz
M band 60 to 100 GHz

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Waveguide frequency bands

Band Frequency range [1]
R band 1.70 to 2.60 GHz
D band 2.20 to 3.30 GHz
S band 2.60 to 3.95 GHz
E band 3.30 to 4.90 GHz
G band 3.95 to 5.85 GHz
F band 4.90 to 7.05 GHz
C band 5.85 to 8.20 GHz
H band 7.05 to 10.10 GHz
X band 8.2 to 12.4 GHz
Ku band 12.4 to 18.0 GHz
K band 15.0 to 26.5 GHz
Ka band 26.5 to 40.0 GHz
Q band 33 to 50 GHz
U band 40 to 60 GHz
V band 50 to 75 GHz
W band 75 to 110 GHz
Y band 325 to 500 GHz

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References

  • 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.

See also

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References

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