In
telecommunication,
free-space path loss (
FSPL) is the loss in
signal strength of an
electromagnetic wave that would result from a
line-of-sight path through free space (usually air), with no obstacles nearby to cause
reflection or
diffraction. It is defined in "Standard Definitions of Terms for Antennas", IEEE Std 145-1983, as "The loss between two isotropic radiators in free space, expressed as a power ratio." Usually it is expressed in dB, although the IEEE standard does not say that. So it assumes that the
antenna gain is a power ratio of 1.0, or 0 dB. It does not include any loss associated with hardware imperfections, or the effects of any antennas gain. A discussion of these losses may be found in the article on
link budget. The
FSPL is rarely used standalone, but rather as a part of the
Friis transmission equation, which includes the gain of antennas.