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. It decreases with the square of the wavelength of the radio waves, and does not include any power loss in the antennas themselves due to imperfections such as resistance or losses due to interaction with the environment such as atmospheric absorption.

The FSPL is rarely used standalone, but rather as a part of the Friis transmission formula, which includes the gain of antennas. It is a major factor used in power link budgets to analyze radio communication systems, to ensure that sufficient radio power reaches the receiver so that the received signal is intelligible.