In telecommunication, radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere and there were no atmospheric anomalies, the radio horizon would be a circle. To compute the radius of the circle drawn on the earth in such a case use the formula:
This is the geometric straight line of sight horizon. For an equivalent formula for an antenna height in metres and a radio horizon in kilometres would be
These formulas are approximations for the case that the height is much smaller than the earth radius. The precise formula is
where r is the earth radius and h the height.
The radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range.
Antenna heights above 1,000,000 feet (189 miles; 305 kilometres) will cover the entire hemisphere and not increase the radio horizon.
Very high frequency(VHF) and Ultra high frequency(UHF) radio signals will bend slightly toward the Earth's surface. This bending effectively increases the radio horizon and therefore slightly increases the formula constant. The American Radio Relay League(ARRL) Antenna Book gives a constant of 1.415 for the feet/miles formula for weak signals during normal tropospheric conditions. This can usefully be approximated as:
In practice, radio wave propagation is affected by atmospheric conditions, ionospheric absorption, and the presence of obstructions, for example mountains or trees. The simple formula above gives a bestcase approximation of the maximum propagation distance but is not adequate for determining the quality of service at any location.
See also
References
 The ARRL Antenna Book, 19th Edition, R. Dean Shaw, pp 22–25
External links
ELF
3 Hz
30 Hz
SLF
30 Hz
300 Hz
ULF
300 Hz
3 kHz
VLF
3 kHz
30 kHz
LF
30 kHz
300 kHz
MF
300 kHz
3 MHz
HF
3 MHz
30 MHz
VHF
30 MHz
300 MHz
UHF
300 MHz
3 GHz
SHF
3 GHz
30 GHz
EHF
30 GHz
300 GHz
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MILSTD188).
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