pulsAR
radio Operator's Manual
Table 6.3 – Antenna heights (feet) to clear the earth and 60% of the Fresnel Zone (2.4 GHz)
Distance (miles)
5
10
20
30
40
50
Antenna height to clear
earth (ft)
3
12
50
113
200
313
60% Fresnel Zone radius
at mid-point (ft)
31
44
62
76
87
98
Total antenna height
required (ft)
34
56
112
189
287
412
6.2.4
Atmospheric Refraction
Under normal atmospheric conditions radio waves do not propagate in a straight line, they actually
bend slightly downward. This is due to "refraction" in the atmosphere that affects radio waves
propagating horizontally. To take this downward bending into account, we perform all the RF path
calculations using a larger value for the earth radius, such that we can then consider the radio waves
as propagating in a straight line.
In the Fresnel Zone calculator you can change the earth radius multiplying factor (the "k factor") to
take into account different atmospheric conditions. Under normal conditions the "k factor" is 4/3.
However unusual weather conditions can cause significant changes to the refraction profile. For a
high reliability link you may want to use a lower value for the k factor.
6.2.5
Clearing Obstructions
The calculator allows you to quickly determine whether you have enough clearance above a particular
obstruction in the RF path, or alternatively, how high you need to elevate your antennas to clear the
obstruction.
For each potential obstruction in the path you need to know its distance from one of your end points
and the height of the obstruction. Drawing the path in “Google Earth” is a quick way of identifying
buildings or structures that lay in the direct path and finding their distance from the end points. You
may need to use a topographic map, draw the line between the end points, and create an accurate
terrain profile. If there are buildings or trees in the path you need to determine or estimate their
height and add it to the terrain elevation at those points.
For each of these potential obstruction points, enter its distance from site 1 and the height of the
obstruction above sea level in the bottom left input “spinners” of the calculator. On the right hand
side the calculator displays the vertical separation between the top of the obstruction and the bottom
of the Fresnel Zone. If this value is negative you can use the antenna height spinners to increase the
height of one or both antennas until that clearance becomes greater than zero.
6-5
Summary of Contents for AR-24010E
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