C-5
Cisco Aironet 350 Series Bridge Hardware Installation Guide
OL-1412-01
Appendix C Antenna Basics
Antenna System
Parabolic dishes have very high gain (typically 21 dBi) along with a very narrow radiation angle
(typically 12.5 degrees) and must be accurately aimed at the other antenna. Yagi antennas have high gain
(typically 13.5 dBi) and a wider radiation angle (typically 25 to 30 degrees). Yagi antennas must also be
properly aimed at the other antenna. Patch antennas have high gain (typically 6 dBi) and a relatively
broad radiation angle. The patch antenna is more tolerant of orientation, but must still be positioned to
face the direction of the other antenna.
Diversity Antennas
A diversity antenna is physically two antennas (sometimes in a single package) that provide diversity
antenna operation. In diversity operation, the radio receives signals on both antennas but responds
(transmits) on the antenna with the strongest received signal. Diversity operation helps improve system
performance when signals are being reflected along different paths to the antenna. Diversity antennas
for the bridge are available in 2.2 dBi and 5.2 dBi gain ratings.
Basic Antenna Alignment
When you are using directional antennas to communicate between two bridges, you must manually align
the antennas for proper bridge operation. Directional antennas have greatly reduced radiation angles.
The radiation angle for yagi antennas is approximately 25 to 30 degrees, and for parabolic dish antennas
the radiation angle is approximately 12.5 degrees.
You can use the bridge link test to help measure the alignment of two antennas after the bridges are
associated. The association indicates the antennas are pointing in the general vicinity of each other, but
does not indicate that the antennas are properly aligned. The link test provides information you can use
to gauge the alignment.
Typically, when two antennas are aligned to the edges of their radiation patterns, communication can be
marginal as indicated by lost packets, high retry counts, and a low signal strength. However, when two
antennas are properly aligned, communication is improved as indicated by all packets being received,
lower retry counts, and a higher signal strength.
The following steps provide general guidelines for basis antenna alignment:
Step 1 Obtain the initial alignment direction. This may involve a visual line of sight for shorter distances or a
Global Positioning System (GPS) receiver, map, and compass for longer distances.
Note The antennas must be located above interfering objects such as trees and buildings.
Step 2 When the antennas are initially aligned, set the bridge for a 1-Mbps data rate and check for association
between the bridges.
a. From the bridge Summary Status home screen, click Setup.
b. Click Root Radio Hardware.
c. In the Data Rates selection boxes for 2.0, 5.5, and 11.0 click each of the pull down arrows and select
no.
d. Click OK on the bottom of the screen, then on the pop-up message click OK to update the
selections.
e. On the bridge Setup screen, click Associations to view the Association Table screen and verify that
the destination bridge is listed in the Association Table.
