User's Manual

AR24027 Operator’s Manual (rev E)
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4.3.3 Antenna Alignment
When mounting the high gain antenna (24 dBi), the proper antenna alignment is extremely important
since the beam-width of the antenna is very narrow. Once you perform a rough alignment and the
link is in operation, you can use the “monitor-link” and “antenna-alignment-aid” commands. Type:
> monitor-link
in order to update, every half second, the link statistics including the RSSI level. The antenna can
then be aligned so that the RSSI is maximized.
Since in many applications the antenna is on a tower where it is not practical to have a terminal
nearby, the AR24027 has an additional “antenna alignment aid” available at the outdoor unit. This
feature uses the three pin “Auxiliary port” connector to output an audio signal with a pitch
proportional to the receive signal strength. AFAR provides a special cable adapter that converts the
three-pin connector into a standard female audio jack. Use this cable to connect the three-pin
connector to a pair of standard headphones while aligning the antenna. At a terminal session issue the
command:
>aaa audio (aaa is an abbreviation for “antenna-alignment-aid”)
and then align the antenna until you hear the highest audio pitch.
Once the antenna is aligned you may type the command:
>aaa off
to turn off the audio signal and revert the auxiliary port connector to console mode.
4.3.4 Spectrum Analysis and channel selection
Radio operation in unlicensed bands has the potential of suffering from interference from other
equipment operating in the same band. The use of directive antennas greatly reduces the potential for
interference. In addition, the AR24027 includes several features, described below, to identify and
overcome sources of interference.
The AR24027 can be commanded to perform a spectrum analysis of the ISM band and report the
results in either a graphical or tabular form. The command:
>spectrum-analysis input=a-antenna dwell=xx
instructs the radio to scan the entire band, dwelling on each channel for a programmable amount of
time, and record the highest signal level in that channel. This feature can be used to perform a site
survey and identify the best receive channel.
Note that even though the AR24027 channels are spaced 2 MHz apart, the receiver RF bandwidth is
approximately 5 MHz. Therefore the RSSI value reported for each channel represents the total
energy in a 5 MHz band centered around that channel. For this reason, a narrow band transmitter will
show up in the spectrum analysis report as a lobe with 5 MHz bandwidth. Conversely, you do not
need to find a quiet 5 MHz wide region in the spectrum analysis report to select a quiet channel, i.e.,
any single channel sample that shows a low “noise” level, is a good candidate to select a s a receive
channel.