Free-Space Path Loss (in dB)


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.

Free-space path loss is proportional to the square of the distance between the transmitter and receiver, and also proportional to the square of the frequency of the radio signal.

Related formulas


FSPLfree-space path loss (dB) (dimensionless)
cSpeed of light
df signal frequency (GHz)