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The formulas below can be used to calculate if the radio link has an acceptable fade margin or, if not, how much antenna gain that needs to be added or if repeaters have to be used.

The known factors are often:

This is a calculation with all the possible losses in the system and subtracting the losses from the line of sight to give an estimated value of your likely link performance.

FM = Srx + Ptx + Gtx + FSL + Grx – Cl

Can be used to calculate any of the factors within.

FM = Fade Margin Srx = Sensitivity of the receiver (dBm) (using +dBm instead of –dBm) Ptx = Transmitter RF output power (dBm)
Gtx = TX Antenna Gain (dB)
FSL = Free Space Loss (dB) 
Grx = Receiver (RX) Antenna Gain (dB)
Cl = Cable/Connector Loss (dB)    

Note! To simplify, we use a general Cable/Connector loss, including both the Tx and Rx site, of 4 dB. This is a high value for most installations, but still assumes a correct antenna cable installation.

Example: Receiver sensitivity needed.

Using an assumption of 3 dB antennas at each site:

FM = Srx + Ptx + Gtx + FSL + Grx – Cl -> Srx = Ptx + Gtx + FSL + Grx – Cl – FM ->  
     
Srx = 33 dBm + 3 dBd – 111.7 dB + 3 dBd – 4 dB – 20 dB = -96.7 dBm   

The following formula below can be used to calculate any of the factors within.

FM = Srx + Ptx + Gtx + A + Grx – Cl

Where

FM = Fade Margin Srx = Sensitivity of the receiver (dBm) (using +dBm instead of –dBm)
Ptx = Transmitter RF output power (dBm)
Gtx = TX Antenna Gain (dB)
A = Over Air Attenuation (dB)
Grx = Receiver (RX) Antenna Gain (dB)
Cl = Cable/Connector Loss (dB)    

Note! To simplify, we use an average Cable/Connector loss value, including both the Tx and Rx site, of 4 dB. This is in some cases a high value, but still assumes a correct antenna cable installation.

Example 1

Is the radio link theoretically possible? Calculation of fade margin FM:

Distance = 3 miles (5 kilometres)
Antenna height 1 = 65 feet (20 meters)
Antenna height 2 = 16 feet (5 meters)
Radio Tx Power = 33 dBm (2 W)
Radio Rx Sensitivity = -110 dBm
Frequency = 456.000 MHz
Antenna Gain 1 = 3 dBd
Antenna Gain 2 = 6 dBd
Cable / Connector losses = 4 dB total 

FM = ?

A = 117 + 40 x log 3 + 20 x log 456 – 20 x log(65 x 16) = 129 dB

FM = Srx + Ptx + Gtx + A + Grx – Cl ->

FM = 110 dBm + 33 dBm + 3 dBd – 129 dB + 6 dBd – 4 dB = 19 dB

The fade margin is 19 dB, which is an acceptable level. The radio link should be possible.