An equation giving the variation in power between a transmitter and receiver in a telecommunication system; this variation is known as path loss. In a system with a transmitter of power P_T, gain G_T, and a receiver of gain G_R, received power P_R, separated by a distance R and operating at a wavelength λ, the Friis transmission equation gives the received power as

This specifies the relation in free space; modifications have to be made if the transmission is influenced by other objects, such as the earth’s surface.