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•
Marginal communications quality in late fall (with no leaves on the trees along the signal path) might fail in the
summer (with leaves on the trees along the signal path).
•
In winter, a wireless link can fail when the antenna is exposed to ice buildup or when the antenna elements are
covered with snow.
Radio paths over water or extremely flat ground require optimization of antenna height at one end. This is due to in-phase or
out-of-phase reflections. Adjustment of antenna height by 1 to 3 meters may move the signal from null to peak.
Long distance links may be obstructed by earth curvature, so the antenna height requirements must not only take the height
of obstructions and Fresnel Zone into account but also the earth bulge. The earth bulge is approximately 5 meters (16.4 ft.) at
a link distance of 16 Kilometers (10 millimeters). Consult your supplier to take appropriate steps to maintain or optimize
wireless link performance.
9.3.1 Fresnel Zone Calculation
The exact shape and width of the Fresnel Zone is determined by calculating the Fresnel Radius. The distance between the
Fresnel Zone boundary and a straight line running along the signal path (shortest path) between the antennas is called the
Fresnel Radius. Fresnel Radius can be determined by using the path difference (difference between the shortest path and
alternative path) and frequency of the radio signal. If there are no obstacles in the space forming 60% of the path difference,
then the propagation characteristics are said to be the same as that in free space.
Let's say, in the above figure:
–
d1 is the distance between the obstacle and the antenna at one end.
–
d2 is the distance between the obstacle and the antenna at the other end.
λ
–
is the wavelength of the operating frequency.
Then, Path Difference (d3) and Fresnel Radius (r
®
ORiNOCO
AP-9200R - Hardware Installation Guide
Figure 9-1 Fresnel Zone Calculation
th
, radius of the n
n
(where n = 1, for the first Fresnel Zone and r
Measuring Signal Performance
Fresnel Zone) can be calculated from the formula below:
r
)
n =
1
54
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