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Radio transmission requires a clear path between antennas
known as radio line of sight. It is necessary to understand the
requirements for radio line of sight when designing a network
operating in the 2.4Ghz ISM band.
Line of sight is the direct free-space path that exists
between two points. Using binoculars on a clear day, it is easy to
determine if visual line of sight exists between two points that
are miles apart. To have a clear line of sight there must be no
obstructions between the two locations. Often this means that the
observation points must be high enough to allow the viewer to see
over any ground-based obstructions.
The following obstructions might obscure a visual
link:
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Topographic
features, such as mountains |
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The curvature
of the Earth |
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Buildings and
other man-made objects |
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Trees |
If any of these
obstructions rise high enough to block the view from end to end,
there is no visual line of sight.
Obstructions
that can interfere with visual line of sight can also interfere
with radio line of sight.
But one must also consider the Fresnel effect. If a
hard object, such as a mountain ridge or building, is too close to
the signal path, it can damage the radio signal or reduce its
strength. This happens even though the obstacle does not obscure
the direct, visual line of sight. The Fresnel zone for a radio
beam is an elliptical area immediately surrounding the visual
path. It varies in thickness depending on the length of the signal
path and the frequency of the signal.
As shown in the
picture above, when a hard object protrudes into the signal path
within the Fresnel zone, knife-edge diffraction can deflect part
of the signal and cause it to reach the receiving antenna slightly
later than the direct signal. Since these deflected signals are
out of phase with the direct signal, they can reduce its power or
cancel it out altogether. If trees or other 'soft' objects
protrude into the Fresnel zone, they can attenuate (reduced the
strength of) a passing signal. In short, the fact that you can see
a location does not mean that you can establish a quality radio
link to that location.
There are
several options to establish or improve the line of sight:
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Raise the antenna mounting point on the existing structure
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Build a new structure, i.e. radio tower, which is tall enough to
mount the antenna
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Increase the height of an existing tower
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Locate a different mounting point, i.e. building or tower, for the
antenna
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Cut down problem trees
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