Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation.
Electromagnetic transmission includes light emissions traveling in a straight line. The rays or
waves may be diffracted, refracted, reflected, or absorbed by atmosphere and obstructions with
material and generally cannot travel over the horizon or behind obstacles.
Line of sight propagation to an antenna
At low frequencies (below approximately 2 MHz or so) radio signals travel as ground waves,
which follow the Earth's curvature due to diffraction with the layers of atmosphere. This enables
AM radio signals in low-noise environments to be received well after the transmitting antenna
has dropped below the horizon. Additionally, frequencies between approximately 1 and 30 MHz,
can be reflected by the F1/F2 Layer, thus giving radio transmissions in this range a potentially
global reach (see shortwave radio), again along multiple deflected straight lines. The effects of
multiple diffraction or reflection lead to macroscopically "quasi-curved paths".
However, at higher frequencies and in lower levels of the atmosphere, neither of these effects are
significant. Thus any obstruction between the transmitting antenna and the receiving antenna will
block the signal, just like the light 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 of highfrequency radio 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.
Contents
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1 Radio horizon
o 1.1 Earth bulge and atmosphere effect
o 1.2 Geometric distance to horizon
o 1.3 The actual service range
o 1.4 Example
2 Line-of-sight propagation as a prerequisite for radio distance measurements
3 Impairments to line-of-sight propagation
4 Mobile telephones
5 See also
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6 References
7 External links
Radio horizon
The 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 atmosphere, the radio
horizon would be a circle.
R is the radius of the Earth, h is the height of the transmitter (exaggerated), d is the line of sight
distance
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.
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:
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 and atmosphere effect
Earth bulge is a term used in telecommunications. It refers to the circular segment of earth
profile which blocks off long distance communications. Since the geometric line of sight passes
at varying heights over the Earth, the propagating radio wave encounters slightly different
propagation conditions over the path. The usual effect of the declining pressure of the
atmosphere with height is to bend radio waves down toward the surface of the Earth, effectively
increasing the Earth's radius, and the distance to the radio horizon, by a factor around 4/3. [1] This
k-factor can change from its average value depending on weather.
Geometric distance to horizon
Assuming a perfect sphere with no terrain irregularity, the distance to horizon from a high
altitude transmitter (i.e., line of sight) can readily be calculated.
Let R be the radius of Earth and h be the altitude of a telecommunication station. Line of sight
distance d of this station is given by the Pythagorean theorem;
Since the altitude of the station is much less than the radius of the Earth,
If the height is given in metres, and distance in kilometres, [2]
If the height is given in feet, and the distance in miles,
The actual service range
The above analysis doesn’t take the effect of atmosphere on the propagation path of the RF
signals into consideration. In fact, the 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 (geometric) distance.
Usually a factor k is used in the equation above
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 [3] to be 4/3. That means that, the maximum
service range increases by % 15
for h in meters and d in km.
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.[4]
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,
Line-of-sight propagation as a prerequisite for radio
distance measurements
Travel time of radio waves between transmitters and receivers can be measured disregarding the
type of propagation. But, generally, travel time only then represents the distance between
transmitter and receiver, when line of sight propagation is the basis for the measurement. This
applies as well to RADAR, to Real Time Locating and to LIDAR.
This rules: Travel time measurements for determining the distance between pairs of transmitters
and receivers generally require line of sight propagation for proper results. Whereas the desire to
have just any type of propagation to enable communication may suffice, this does never coincide
with the requirement to have strictly line of sight at least temporarily as the means to obtain
properly measured distances. However, the travel time measurement may be always biased by
multi-path propagation including line of sight propagation as well as non line of sight
propagation in any random share. A qualified system for measuring the distance between
transmitters and receivers must take this phenomenon into account. Thus filtering signals
traveling along various paths makes the approach either operationally sound or just tediously
irritating.
Impairments to line-of-sight propagation
Two stations not in line-of-sight may be able to communicate through an intermediate radio
repeater station.
Low-powered microwave transmitters can be foiled by tree branches, or even heavy rain or
snow.
If a direct visual fix cannot be taken, it is important to take into account the curvature of the
Earth when calculating line-of-sight from maps.
The presence of objects not in the direct visual line of sight can interfere with radio transmission.
This is caused by diffraction effects: for the best propagation, a volume known as the first
Fresnel zone should be kept free of obstructions.
Objects within the Fresnel zone can disturb line of sight propagation even if they don't block the
geometric line between antennas
Reflected radiation from the ground plane also acts to cancel out the direct signal. This effect,
combined with the free-space r−2 propagation loss to a r−4 propagation loss. 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.
Mobile telephones
Although the frequencies used by mobile 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:
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r−4 propagation over the rooftop landscape
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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. For mobile phone services these
problems are tackled using:
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rooftop or hilltop positioning of base stations
many base stations (a phone can typically see six at any given time)
rapid handoff between base stations (roaming)
extensive error correction and detection in the radio link
sufficient operation of mobile phone in tunnels when supported by split cable antennas
local repeaters inside complex vehicles or buildings
Other conditions may physically disrupt the connection surprisingly without prior notice:
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local failure when using the mobile phone in buildings of concrete with steel
reinforcement
temporal failure inside metal constructions as elevator cabins, trains, cars, ships
See also
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Anomalous propagation
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