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WAB–3000 Wireless Access Point
Where
L
= free space path loss between antennas
P
F = frequency in GHz
D = path length in miles
Bridge Antenna Location
When as bridge device, the WAB–3000 may need to be mounted out-
doors on a high place to achieve the best bridge result. The Fresnel zone
and Earth bulge dominate to decide how high that the unit's Antenna
need be put. The total antenna height equals the width of Fresnel zone
plus the height of earth bulge.
The Fresnel zone is the area around the visual line-of-sight that radio
waves spread out into after they leave the antenna. This area must be
clear or else signal strength will weaken. The rule of thumb is that 60% of
the Fresnel zone must be clear of obstacles. Typically, 20% Fresnel Zone
blockage introduces little signal loss to the link. Beyond 40% blockage,
signal loss will become significant.
The equation of the width of Fresnel Zone is:
Where
W = Width of the Fresnel Zone (in feet)
D = Distance between the antennas (in miles)
F = Frequency in GHz
When the transmit distance of RF signal is longer than seven miles,
the curvature of the earth may be a factor and require the antenna put at
higher location. The additional antenna height can be calculated by below
formula:
Where
H = Height of earth bulge (in feet)
D = Distance between antennas (in miles)
29000171-002 A
Chapter 2: Hardware Installation
17
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