The ITU Radiocommunication Assembly, considering that in a radio link between a transmitter and a receiver, the ratio between the power supplied by the transmitter and the power available at the receiver input depends on several factors such as the losses in the antennas or in the transmission feed lines, the attenuation due to the propagation mechanisms, the losses due to faulty adjustment of the impedances or polarization, etc.
ITU-R P.341-5 FRENCH-1999 Referenced Document
ITU-R P.525 Calculation of free-space attenuation*， 2019-08-01 Update