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8
Aligning Antennas
After wireless bridge units have been mounted, connected, and their radios are operating, bridge link
antennas must be accurately aligned to ensure optimum performance. This alignment process is
particularly important for long-range point-to-point links. In a point-to-multipoint configuration the root
bridge uses an omnidirectional or sector antenna, which does not require alignment, but bridge nodes
still need to be correctly aligned with the root bridge antenna.
•
Point-to-Point Configurations
– In a point-to-point configuration, the alignment process
requires two people, one at each end of the link. The use of cell phones or two-way radio
communication may help with coordination. To start, you can just point the antennas at each
other, using binoculars or a compass to set the general direction. For accurate alignment, you
must monitor the signal strength LEDs as the antenna moves horizontally and vertically.
•
Point-to-Multipoint Configurations
– In a point-to-multipoint configuration all bridge
nodes must be aligned with the root bridge antenna. The alignment process is the same as in
point-to-point links, but only the bridge node end of the link requires the alignment.
The signal strength LEDs indicate the received radio signal strength for a particular bridge link. The more
LEDs that turn on, the stronger the signal. Alternatively, you can monitor the Receive Signal Strength
Indicator (RSSI) value directly from the management interface. The higher the RSSI value, the stronger
the signal.
When you move the antenna during alignment, the radio signal from the remote antenna can be seen
to have a strong central main lobe and smaller side lobes. The object of the alignment process is to set
the antenna so that it is receiving the strongest signal from the central main lobe.
To align the antennas in the link, monitor the signal strength LEDs or the RSSI value in the management
interface. Start with one antenna fixed and then perform the following procedure on the other antenna:
Main Lobe
Maximum
Horizontal Scan
Vertical Scan
RSSI
Value
Side Lobe
Maximum
RSSI Value
Remote
Antenna
Maximum Signal Strength Position
for Horizontal Alignment
Maximum Signal
Strength Position for
Vertical Alignment
