Free-space path loss
In telecommunications, the free-space path loss (FSPL) (also known as free-space loss, FSL) is the decrease in signal strength of a signal traveling between two antennas on a line-of-sight path through free space, which occurs because the signal spreads out as it propagates. The "Standard Definitions of Terms for Antennas", IEEE Std 145-1993, defines free-space loss as "The loss between two isotropic radiators in free space, expressed as a power ratio." Free-space path loss increases with the square of the distance between the antennas because radio waves spread out following an inverse square law.