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EtherWave - WMR
7.0 Installation
7.1 Path Calculation
Assuming adequate antenna heights, a basic formula to determine if
an adequate radio signal path exists (i.e. there is a reasonable fade
margin to ensure reliability) is:
Fade Margin = System Gain - Path Loss
where all values are expressed in dB.
As discussed on the previous page, a desired fade margin is 20dB.
System gain is calculated as follows:
System Gain = Transmitter Power + (Transmitter Antenna
Gain -
Transmitter Cable and Connector
Losses) + (Receiver Antenna Gain - Re
ceiver Cable and Connector Losses) +
| Receiver Sensitivity |.
where all values are expressed in dB, dBi, or dBm, as applicable.
Assuming a path loss of 113dB for this example, the fade margin =
143-113 = 30dB.
30dB exceeds the desired fade margin of 20dB, therefore this radio
communications link would be very reliable and robust.
On the following page are examples of actual path loss measure-
ments taken in an open rural environment; the path loss numbers do
not apply to urban or non-LOS environments.
FCC regulations allow for up
to 36dBi effective isotropic
radiated power (EIRP). The
sum (in dBm) of the
transmitted power, the
cabling loss, and the
antenna gain cannot exceed
36dBi.
System Gain = 30+(6-2)+(3-2)
+105
= 30+4+1+105
= 140dB.
Example 7.1.1:
Tx power = 30dBm
Tx antenna gain = 6dBi
Tx cable/connector loss = 2dB
Rx antenna gain = 3dBi
Rx cable/connector loss = 2dB
Rx sensitivity = -105dBm
