User's Manual

AR24027 Operator’s Manual (rev E)
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Calculate link margin by subtracting radio sensitivity from the calculated RSS level.
Calculated RSS level at receiver = -62 dBm
Sensitivity of 2411 at 11 Mbps = -81 dB
link margin = +19 dB
This figure, link margin, is the amount of signal received by the radio that is above the minimum
required for the radio to meet its performance characteristics. This value is important since it gives
the operator an indication of how much signal fade the system can tolerate. Signal fading may be
caused by multiple radio paths (reflections) atmospheric conditions such as rain, temperature
inversions, fog, etc., and may last anywhere from a few moments to several hours and cause as much
as 20 to 30 dB of signal strength loss. Although it is possible to operate a system with a link margin
as low as 5 dB, as general rule of thumb it is recommended that all systems have a link margin of
better than 20 dB.
3.3.4 Antenna Orientation
Antennas at each end of a communications link must be mounted similarly in terms of polarity, and
directional antennas must be carefully oriented towards each other. The choice of polarization –
horizontal vs. vertical – is in many cases arbitrary. However, interfering signals from such devices as
cellular phones and pagers are generally polarized vertically, and an excellent means of reducing their
effect is to mount system antennas for horizontal polarization. Of those antennas in section 3.2 for
AR24027 systems, the directional antennas can be mounted for horizontal or vertical polarization,
while the omnidirectional antennas use only vertical polarization. Horizontally polarized
omnidirectional antennas are available as a special purchase.
Orientation of directional antennas is critical because their sensitivity is greatly reduced outside a
fairly narrow angle. Performance of the system can be seriously degraded by mis-aligned directional
antennas. The AR24027 has a built in feature that allows the operator to use an audio to assist in
aligning the antenna. Refer to chapter 5 on the use of this built-in antenna alignment feature.
3.3.5 Cable Loss (Attenuation)
The AR24027 is housed in a watertight enclosure so that it may be mounted in very close proximity
to the antenna. Using short cables to connect the radio to the antenna reduces signal losses. Table 3.2
shows loss per 100 feet (30 meters) at 2.4 GHz for typical antenna cable types.
RSS = 38 dBm - 124 dB + 24 dB
= -62 dBm