Line-of-sight propagation is a characteristic ofelectromagnetic radiation or acousticwave propagation which means waves can only travel in a direct visual path from the source to the receiver without obstacles.^[1] Electromagnetictransmission includes light emissions traveling in astraight line. The rays or waves may bediffracted,refracted, reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over thehorizon or behind obstacles.

In contrast to line-of-sight propagation, atlow frequency (below approximately 3 MHz) due todiffraction,radio waves can travel asground waves, which follow the contour of the Earth. This enablesAM radio stations to transmit beyond the horizon. Additionally, frequencies in theshortwave bands between approximately 1 and 30 MHz, can be refracted back to Earth by theionosphere, calledskywave 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) and the receiving antenna (receiver) will block the signal, just like thelight 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). BroadcastFM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.

Low-poweredmicrowave 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 firstFresnel zone should be free of obstructions.

Reflected radiation from thesurface 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 asheight gain.

See alsoNon-line-of-sight propagation 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 used4⁄3 Earth radius to compute clearances along the path.

Although the frequencies used bymobile phones (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:

• 1⁄r ⁴ propagation over the rooftop landscape^{[clarification needed]}
• 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, withmultipath effects and extensiveRayleigh fading. For mobile phone services, these problems are tackled using:

• rooftop or hilltop positioning of base stations
• manybase 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 withomnidirectional 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 thesignal-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.
• rapidhandoff between base stations (roaming)
• the radio link used by the phones is a digital link with extensiveerror correction and detection in the digital protocol
• sufficient operation of mobile phone in tunnels when supported by split cable antennas
• local repeaters inside complex vehicles or buildings

AFaraday 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.

Theradio horizon is thelocus of points at which direct rays from anantenna are tangential to the surface of the Earth. If the Earth were a perfect sphere without an atmosphere, theradio 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, and the presence of obstructions, for example mountains or trees. Simple formulas that include the effect of the atmosphere give the range as:

${\displaystyle {\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}\mathrm{o}\mathrm{n}}_{\mathrm{m}\mathrm{i}}\approx 1.23\cdot \sqrt{{\mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{f}\mathrm{e}\mathrm{e}\mathrm{t}}}}$

${\displaystyle {\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}\mathrm{o}\mathrm{n}}_{\mathrm{k}\mathrm{m}}\approx 3.57\cdot \sqrt{{\mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{s}}}}$

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.

Intelecommunications,Earth bulge refers to the effect ofearth's curvature 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.^{[citation needed]}

Assuming a perfect sphere with no terrain irregularity, the distance to the horizon from a high altitudetransmitter (i.e., line of sight) can readily be calculated.

LetR be the radius of the Earth andh be the altitude of a telecommunication station. The line of sight distanced of this station is given by thePythagorean theorem;

${\displaystyle d^2=(R+h{)}^2-R^2=2\cdot R\cdot h+h^2}$

The altitude of the stationh is much smaller than the radius of the EarthR. Therefore,${\displaystyle h^2}$ can be neglected compared with${\displaystyle 2\cdot R\cdot h}$.

Thus:

${\displaystyle d\approx \sqrt{2\cdot R\cdot h}}$

If the heighth is given in metres, and distanced in kilometres,^[2]

${\displaystyle d\approx 3.57\cdot \sqrt{h}}$

If the heighth is given in feet, and the distanced in statute miles,

${\displaystyle d\approx 1.23\cdot \sqrt{h}}$

In the case, when there are two stations involve, e.g. a transmit station on ground with a station heighth and a receive station in the air with a station heightH, the line of sight distance can be calculated as follows:

${\displaystyle d\approx \sqrt{2R}\left(\sqrt{h}+\sqrt{H}\right)}$

The usual effect of the declining pressure of the atmosphere with height (vertical pressure variation) is to bend (refract) radio waves down towards the surface of the Earth. This results in aneffective Earth radius,^[3] increased by a factor around4⁄3.^[4] Thisk-factor can change from its average value depending on weather.

The previous vacuum distance analysis does not consider the effect of atmosphere on the propagation path of RF signals. In fact, RF signals do not 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 factork is used in the equation above, modified to be

${\displaystyle d\approx \sqrt{2\cdot k\cdot R\cdot h}}$

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^[5] to be4⁄3. That means that the maximum service range increases by 15%.

${\displaystyle d\approx 4.12\cdot \sqrt{h}}$

forh in metres andd in kilometres; or

${\displaystyle d\approx 1.41\cdot \sqrt{h}}$

forh in feet andd in miles.

But in stormy weather,k may decrease to causefading in transmission. (In extreme casesk can be less than 1.) That is equivalent to a hypothetical decrease in Earth radius and an increase of Earth bulge.^[6]

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,

${\displaystyle d\approx 4.12\cdot \sqrt{1500}=160\text{ km.}}$

• Anomalous propagation
• Dipole field strength in free space
• Knife-edge effect
• Multilateration
• Non-line-of-sight propagation
• Over-the-horizon radar
• Radial (radio)
• Rician fading, stochastic model of line-of-sight propagation
• Slant range

•  This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).

• http://web.telia.com/~u85920178/data/pathlos.htm#bulges
• Article on the importance of Line Of Sight for UHF reception
• Attenuation Levels Through Roofs
• Approximating 2-Ray Model by using Binomial series by Matthew Bazajian

• Radio frequency propagation
• IEEE 802.11

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