Where –

d = Distance between the antennas.

f = Frequency

G (Tx) = The Gain of the Transmitting Antenna.

G (Rx) = The Gain of the Receiving Antenna.

c = Speed of light in vacuum ( Meters per Second)

Free Space Path Loss

The free space path loss is used to predict the strength of a RF signal at a particular distance. This is a theoretical value, as in the real world, there are many obstacles, reflections and losses which need to be accounted for when estimating the signal at a location. However the FSPL is a good approximation for estimating the loss of signal when propagating through free space.

In the above calculation, the free space patch loss calculator takes in to account the gain on both the receiving and transmitting antennas. The gain of the antennas offsets the loss by a certain decibel value. If you ignore the gain at either end i.e the transmitting and receiving end, then the FSPL would just be a factor of the frequency and distance. We have also assumed that the rf signals will be propagating in air. If they were propagating in another medium this calculator would not give the correct result.