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minimize damage from lightning strikes. A lightning arrestor, properly grounded
and installed at each building in the topology, will protect networking equipment
in the building, as well as the people working there, from lightning-induced
surges that travel on Ethernet cables.
A
LIGNMENT
Position bridges so that they are aimed at each other wherever possible. While
maintaining a direct line of sight between antennas helps to ensure a strong
signal, it is not strictly necessary, nor is it always possible. Conditions such as long
distances, mountainous regions, and architectural barriers could make a direct
line of sight nearly impossible to achieve.
In a campus setting, where buildings are short distances apart, and especially in a
point-to-point configuration, align each unit to point at the antenna with which it
will communicate. If you place two units at different heights, tilt them up or
down toward each other for optimal signal strength.
P
OLARIZATION
Polarization is a physical phenomenon of radio signal propagation. In general, any
two antennas that are to communicate with each other must be set for the same
horizontal or vertical polarization. If polarization on both antennas does not
match—a situation called
cross-polarization
—the link will either work poorly or
not at all. Follow these polarization guidelines:
■
Vertical polarization is preferred for point-to-multipoint configuration.
■
For point-to-multipoint configurations, the omnidirectional antenna
connected to the access point should be vertically aligned in relation to
the ground.
■
Unidirectional antennas that link with omnidirectional antennas should always
be oriented for vertical polarization.
R
ESTRICTIONS
ON
A
NTENNA
U
SE
The following restrictions apply to the use of the bridge’s 18 dBi antenna:
■ FCC regulations require that in the United States, use of channels 12 and 13 in
conjunction with an 18 dBi antenna is allowed only at very reduced power,
which the 3Com unit sets automatically.
■ The highest allowable power level is set automatically when the country
selection is made from the pulldown menu in the configuration tool.
■ To work properly with 18 dBi antennas, bridges must be separated by at least
61 Meters (200 Feet).