Free-space loss

In telecommunication, free-space path loss (FSPL) is the attenuation of radio energy between the feedpoints of two antennas that results from the combination of the receiving antenna's capture area plus the obstacle free, line-of-sight path through free space (usually air). 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." Despite this name and definition, the FSPL includes a receiving antenna aperture component in the total attenuation. It does not include any loss associated with hardware imperfections, or the effects of any antenna gains. 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 formula, which includes the gain of antennas.

The free-space path loss (FSPL) formula derives from the Friis transmission formula that states power gain of an antenna system thus...

The FSPL formula expresses a loss value that is the reciprocal of gain and assumes the directivity for the transmit and receive antennas are isotropic and therefore unity. Both modifications simplify the equation to...

where:

Substitute ${\displaystyle c/f}$ for ${\displaystyle \lambda }$ to calculate from frequency where:

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