Path Loss Notes

Free Space Path Loss (FSPL) depends on two items: The first is the frequency of radio signal (f); the second is the wireless transmission distance (d). The following formula highlights the relationship between them.

FSPL (dB) = 20log10(d) + 20log10(f) + K

d = distance

f = frequency

K= constant that depends on the units used for d and f

Here is a table to select K based on units of d and f:

For example, if d is measured in kilometers, f in MHz, the formula is:

FSPL (dB) = 20log10(d)+ 20log10(f) + 32.44

From the Fade Margin equation, Free Space Path Loss can be calculated with the following equation:

Free Space Path Loss (dB)=Tx Power(dBm)-Tx Cable Loss(dB)+Tx Antenna Gain(dB)+Rx Antenna Gain(dB) - Rx Cable Loss(dB) - Rx Sensitivity(dBm) - Fade Margin(dB)

Assume a desired fade margin of for example 12 dB and use actual equipment specifications to determine whether reliable communications can take place. A surplus signal power above the Free Space Path Loss, taking the equipment specs into account, means at least the minimum threshold for reliable communications has been met.