Editing Free-space path loss (section)

{{Short description|Path loss of radio transmitted through air or vacuum}}
{{Use American English|date = March 2019}}

In [[telecommunications]], the '''free-space path loss''' ('''FSPL''') (also known as free-space loss, FSL) 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 propagation|line-of-sight]] (LoS) path through [[Vacuum|free space]] (usually air).<ref name="SensorsLowPower">{{cite book|last1=Islam|first1=Syad Kamrul|last2=Haider|first2=Mohammad Rafiqul|title=Sensors and Low Power Signal Processing|date=10 December 2009 |isbn=978-0387793917|page=49|publisher=Springer |edition=2010}}</ref>  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."<ref name="IEEE Std 145-1993(R2004)">{{cite book|title=IEEE Std 145-1993(R2004), IEEE Standard Definitions of Terms for Antennas|date=1993|publisher=The Institute of Electrical and Electronics Engineers, Inc.|location=New York, NY|isbn=1-55937-317-2|page=14}}</ref>  It does not include any power loss in the antennas themselves due to imperfections such as resistance.   Free-space loss increases with the square of distance between the antennas because the radio waves spread out by the [[inverse square law]] and decreases with the square of the [[wavelength]] of the radio waves.   The FSPL is rarely used standalone, but rather as a part of the [[Friis transmission equation|Friis transmission formula]], which includes the gain of antennas.<ref name="Friis">{{cite journal|last1=Friis|first1=H.T.|title=A Note on a Simple Transmission Formula|journal=IRE Proc.|date=May 1946|volume=34 |issue=5 |pages=254–256|doi=10.1109/JRPROC.1946.234568 |s2cid=51630329 }}</ref>  It is a factor that must be included in the power [[link budget]] of a radio communication system, to ensure that sufficient radio power reaches the receiver such that the transmitted signal is received intelligibly.