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As a signal spreads out from a radiating source, the energy spreads out over a larger surface area.
As this occurs, the strength of that signal gets weaker. FSL specifies how much the signal has
weakened over a given distance, and it is measured in dB.
I.2. RF Path Loss and Transmission Distance Calculation
System Gain (dBm) is the is the total gain of radio without antenna/cable
System Gain = Tx power – Rx Sensitivity
FSL = Tx Power + Tx Antenna Gain + Rx Antenna Gain– Rx Sensitivity
= 32.4 + 20log
10
f
(MHz) + 20log
10
d
(km)
= 36.4 + 20log
10
f
(MHz) + 20log
10
d
(mile)
= 92.4 + 20log
10
f
(GHz) + 20log
10
d
(km)
= 96.4 + 20log
10
f
(GHz) + 20log
10
d
(mile)
(
f
: radio frequency
d
: distance between the transmitter and the receiver)
Fade Margin is an
“
extra” signal power added to ensure the proper working of a link.
Fade Margin = System Gain + Antenna Gain – FSL – Cable Loss
A zero Fade Margin indicates the ultimate connection between the transmitter and receiver system. A
larger Fade Margin indicates a stronger signal for connection, and a negative Fade Margin indicates
connection fail.
Here is an example of EKI-6322AG with 12 dBi antenna (neglect the cable loss in this case).
From the specification of EKI-6321AG:
Transmission power : 14 dBm
Receiver sensitivity : -74 dBm
Antenna gain : 12 dBi
Frequency of 802.11b/g : 2.4GHz
According to the specification, we can calculate the Free Space Loss :
FSL = 14 + (12 +12) – (-74) = 112
The ultimate distance between transmit and receive antennas can also be estimated:
FSL = 112 = 32.4 + 20log
10
(2400) + 20log
10
d
(km)
d
= 3.08 km
