I-O Wireless WBC11, Руководство пользователя

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2.1.4 Line of sight
Radio signals generally require a clear path between antennas. It is necessary to
know the requirements of a clear path (known as ‘line of sight’).
Line of Sight (LOS) means a bit more than the words say. Of course the first
condition for a LOS link is that the two antennas can be connected with an
imaginary straight string. There should never be any object blocking this line.
Achieving this with a short link is fairly easy. Long links, however, can cause
problems. As well as binoculars and a compass, a large calibrated tether balloon
can be used for determining line of site. Letting this balloon up on one side until it
is visible from the other side will tell you exactly the required height for this link.
Repeat this for both sides.
Besides the LOS there is the Fresnel zone. This zone 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. The necessary
clearance for the Fresnel zone can be calculated, and must be take in account
when determining the height of the antennas.
Figure 2.2 Line of Sight and the Fresnel Zone
A rule of thumb for the clearance needed above and below the signal path (H2) is
that for every 1.5 km, a clearance of 4 meter is necessary. A result of this rule is
that large distances need great heights.