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Line-of-sight propagation is a characteristic of
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...
or acoustic
wave propagation Wave propagation is any of the ways in which waves travel. Single wave propagation can be calculated by 2nd order wave equation ( standing wavefield) or 1st order one-way wave equation. With respect to the direction of the oscillation relative to ...
which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a
straight line In geometry, a line is an infinitely long object with no width, depth, or curvature. Thus, lines are one-dimensional objects, though they may exist in two, three, or higher dimension spaces. The word ''line'' may also refer to a line segmen ...
. The rays or waves may be
diffracted Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
,
refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
, reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles. In contrast to line-of-sight propagation, at low frequency (below approximately 3 
MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
) due to diffraction, radio waves can travel as
ground wave Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in rad ...
s, which follow the contour of the Earth. This enables
AM radio AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmis ...
stations to transmit beyond the horizon. Additionally, frequencies in the shortwave bands between approximately 1 and 30 MHz, can be refracted back to Earth by the ionosphere, called skywave or "skip" propagation, thus giving radio transmissions in this range a potentially global reach. However, at frequencies above 30 MHz ( VHF and higher) and in lower levels of the atmosphere, neither of these effects are significant. Thus, any obstruction between the transmitting antenna (
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
) and the receiving antenna ( receiver) will block the signal, just like the
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic at these frequencies is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon". In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.


Impairments to line-of-sight propagation

Low-powered
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
transmitters can be foiled by tree branches, or even heavy rain or snow. The presence of objects not in the direct line-of-sight can cause diffraction effects that disrupt radio transmissions. For the best propagation, a volume known as the first
Fresnel zone A Fresnel zone ( ), named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitter and a receiver. The primary wave will travel in a relative straight line fro ...
should be free of obstructions. Reflected radiation from the surface of the surrounding ground or salt water can also either cancel out or enhance the direct signal. This effect can be reduced by raising either or both antennas further from the ground: The reduction in loss achieved is known as ''height gain''. See also
Non-line-of-sight propagation Non-line-of-sight (NLOS) radio propagation occurs outside of the typical line-of-sight (LOS) between the transmitter and receiver, such as in ground reflections. Near-line-of-sight (also NLOS) conditions refer to partial obstruction by a physica ...
for more on impairments in propagation. It is important to take into account the curvature of the Earth for calculation of line-of-sight paths from maps, when a direct visual fix cannot be made. Designs for microwave formerly used  earth radius to compute clearances along the path.


Mobile telephones

Although the frequencies used by
mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whi ...
s (cell phones) are in the line-of-sight range, they still function in cities. This is made possible by a combination of the following effects: * propagation over the rooftop landscape * diffraction into the "street canyon" below * multipath reflection along the street * diffraction through windows, and attenuated passage through walls, into the building * reflection, diffraction, and attenuated passage through internal walls, floors and ceilings within the building The combination of all these effects makes the mobile phone propagation environment highly complex, with multipath effects and extensive
Rayleigh fading Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices. Rayleigh fading models assume that the magnitude of a signal that has passed through such a transmission m ...
. For mobile phone services, these problems are tackled using: * rooftop or hilltop positioning of base stations * many base stations (usually called "cell sites"). A phone can typically see at least three, and usually as many as six at any given time. * "sectorized" antennas at the base stations. Instead of one antenna with
omnidirectional Omnidirectional refers to the notion of existing in every direction. Omnidirectional devices include: * Omnidirectional antenna, an antenna that radiates equally in all directions * VHF omnidirectional range, a type of radio navigation system for ...
coverage, the station may use as few as 3 (rural areas with few customers) or as many as 32 separate antennas, each covering a portion of the circular coverage. This allows the base station to use a directional antenna that is pointing at the user, which improves the signal-to-noise ratio. If the user moves (perhaps by walking or driving) from one antenna sector to another, the base station automatically selects the proper antenna. * rapid
handoff In cellular telecommunications, handover, or handoff, is the process of transferring an ongoing call or data session from one channel connected to the core network to another channel. In satellite communications it is the process of transf ...
between base stations (roaming) * the radio link used by the phones is a digital link with extensive
error correction and detection In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communica ...
in the digital protocol * sufficient operation of mobile phone in tunnels when supported by split cable antennas * local repeaters inside complex vehicles or buildings A Faraday cage is composed of a conductor that completely surrounds an area on all sides, top, and bottom. Electromagnetic radiation is blocked where the wavelength is longer than any gaps. For example, mobile telephone signals are blocked in windowless metal enclosures that approximate a Faraday cage, such as elevator cabins, and parts of trains, cars, and ships. The same problem can affect signals in buildings with extensive steel reinforcement.


Radio horizon

The ''radio horizon'' is the
locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** ''Locus Award' ...
of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere without an atmosphere, the
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...
horizon would be a circle. The radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. Radio wave propagation is affected by atmospheric conditions,
ionospheric absorption Ionospheric absorption (or ISAB) is the scientific name for absorption occurring as a result of the interaction between various types of electromagnetic waves and the free electrons in the ionosphere, which can interfere with radio transmissions. ...
, and the presence of obstructions, for example mountains or trees. Simple formulas that include the effect of the atmosphere give the range as: :\mathrm_\mathrm \approx 1.23 \cdot \sqrt :\mathrm_\mathrm \approx 3.57 \cdot \sqrt The simple formulas give a best-case approximation of the maximum propagation distance, but are not sufficient to estimate the quality of service at any location.


Earth bulge

In
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
s, Earth bulge refers to the effect of
earth's curvature Spherical Earth or Earth's curvature refers to the approximation of figure of the Earth as a sphere. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Greek philosophers. ...
on radio propagation. It is a consequence of a circular segment of earth profile that blocks off long-distance communications. Since the vacuum line of sight passes at varying heights over the Earth, the propagating radio wave encounters slightly different propagation conditions over the path.


Vacuum distance to horizon

Assuming a perfect sphere with no terrain irregularity, the distance to the horizon from a high altitude
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
(i.e., line of sight) can readily be calculated. Let ''R'' be the radius of the Earth and ''h'' be the altitude of a telecommunication station. The line of sight distance ''d'' of this station is given by the Pythagorean theorem; : d^2=(R+h)^-R^2= 2\cdot R \cdot h +h^2 Since the altitude of the station is much less than the radius of the Earth, : d \approx \sqrt If the height is given in metres, and distance in kilometres, : d \approx 3.57 \cdot \sqrt If the height is given in feet, and the distance in statute miles, : d \approx 1.23 \cdot \sqrt


Atmospheric refraction

The usual effect of the declining pressure of the atmosphere with height (
vertical pressure variation Vertical pressure variation is the variation in pressure as a function of elevation. Depending on the fluid in question and the context being referred to, it may also vary significantly in dimensions perpendicular to elevation as well, and these var ...
) is to bend (
refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
) radio waves down towards the surface of the Earth. This results in an effective Earth radius, increased by a factor around . This ''k''-factor can change from its average value depending on weather.


Refracted distance to horizon

The previous vacuum distance analysis does not consider the effect of atmosphere on the propagation path of RF signals. In fact, RF signals don’t propagate in straight lines: Because of the refractive effects of atmospheric layers, the propagation paths are somewhat curved. Thus, the maximum service range of the station is not equal to the line of sight vacuum distance. Usually, a factor ''k'' is used in the equation above, modified to be : d \approx \sqrt ''k'' > 1 means geometrically reduced bulge and a longer service range. On the other hand, ''k'' < 1 means a shorter service range. Under normal weather conditions, ''k'' is usually chosen to be . That means that the maximum service range increases by 15%. : d \approx 4.12 \cdot \sqrt for ''h'' in metres and ''d'' in kilometres; or : d \approx 1.41 \cdot\sqrt for ''h'' in feet and ''d'' in miles. But in stormy weather, ''k'' may decrease to cause fading in transmission. (In extreme cases ''k'' can be less than 1.) That is equivalent to a hypothetical decrease in Earth radius and an increase of Earth bulge.This analysis is for high altitude to sea level reception. In microwave radio link chains, both stations are at high altitudes. For example, in normal weather conditions, the service range of a station at an altitude of 1500 m with respect to receivers at sea level can be found as, : d \approx 4.12 \cdot \sqrt = 160 \mbox


See also

*
Anomalous propagation Anomalous propagation (sometimes shortened to anaprop or anoprop) Peter Meischner (ed.), ''Weather Radar: Principles and Advanced Applications'', Springer Science & Business Media, 2005, page 144 includes different forms of radio propagation due to ...
* Field strength in free space *
Knife-edge effect Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
*
Multilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multilateration, for e ...
*
Non-line-of-sight propagation Non-line-of-sight (NLOS) radio propagation occurs outside of the typical line-of-sight (LOS) between the transmitter and receiver, such as in ground reflections. Near-line-of-sight (also NLOS) conditions refer to partial obstruction by a physica ...
*
Over-the-horizon radar Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is ...
*
Radial (radio) In RF engineering, radial has three distinct meanings, both referring to lines which radiate from (or intersect at) a radio antenna, but neither meaning is related to the other. ;Ground system radial wires When used in the context of antenna con ...
* Rician fading, stochastic model of line-of-sight propagation *
Slant range In radio electronics, especially radar terminology, slant range or slant distance is the distance along the relative direction between two points. If the two points are at the same level (relative to a specific datum), the slant distance equals t ...


References

*


External links

* http://web.telia.com/~u85920178/data/pathlos.htm#bulges
Article on the importance of Line Of Sight for UHF reception


* ttps://web.archive.org/web/20100921070856/http://techsonar.com/license_4.html Approximating 2-Ray Model by using Binomial series by Matthew Bazajian {{RF Propagation Navbox Radio frequency propagation IEEE 802.11