Afar AR24027, Operator'S Manual

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AR24027 Operator’s Manual
6-12
2. Calculate effective radiated power (ERP) at the transmit antenna using the formula:
n AntennaGai CableLoss TxPower ERP
+ − =
Since the AR24027 is housed in an outdoor enclosure, there is usually little or negligeable
cable loss if the antenna is connected directly to the radio connector. However, if an
additional cable is used between the radio and the antenna, the cable loss (attenuation) must
be included.
Transmit output power = + 23 dBm
Cable and connector loss = - 2 dB
Transmit antenna gain = + 18 dB
Effective Radiated Power = + 39 dBm
NOTE: Table 3.2 lists attenuation values for various cables.
3. Calculate the RSS level at the receive radio using the formula:
Where:
ERP = Effective Radiated Powerfrom the transmit antenna
Lp = Path loss
Gr = Gain of the receive antenna
Cl = Cable and connector losses between antenna and receive radio
For the above system, at a distance of 10 miles, Effective Radiated Power of 39 dBm,
receive antenna gain of 24 dB, and cable loss of 2 dB, the equation would be:
4. Calculate link margin by subtracting radio sensitivity from the calculated RSS level.
Calculated RSS level at receiver = -63 dBm
Sensitivity of AR24027 at 2.7 Mbps = -90 dBm
link margin = +27 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 a general rule of thumb it is recommended that all systems have a link margin of
better than 15 dB.
RSS = ERP - Lp + Gr - Cl
RSS = 39 dBm - 124 dB + 24 dB – 2dB
= -63 dBm