© Microhard Systems Inc.
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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 Ca-
ble and Connector Losses) + (Receiver Antenna Gain - Receiver
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 measurements 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)+108]dB
= [30+4+1+108]dB
= 143dB.
Example:
Tx power = 30dBm
Tx antenna gain = 6dBi
Tx cable/connector loss = 2dB
Rx antenna gain = 3dBi
Rx cable/connector loss = 2dB
Rx sensitivity = -108dBm
