Hi

Free space means without any gas considered along the radio path in other words "free space"=vacuum.

Then if you want to consider influences of atmosphere over your radio path you need to add the attenuation given by gas, rains, snow, water vapor...etc.

All information are available in the ITU recommendations

I have selected some of them below

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.341-5-199910-I!!PDF-E.pdf

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.525-2-199408-I!!PDF-E.pdf

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.676-10-201309-I!!PDF-E.pdf

Radio planners consider a standard atmosphere to dimension radio path. See :

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.835-5-201202-I!!PDF-E.pdf

All ITU recommendations related to propagation (P Series) are available here

                http://www.itu.int/rec/R-REC-P/en

Hope those documents will clarify

Best Regards

Dear Tahir,

please check the Friss Transmission formulae under all operating conditions and media.

its media and distance dependent.

best wishes

What is the difference between friis formula in air and vacuum conditions ?

We need to consider dielectric constant and permittivity of the medium. In Vacuum, these are ideal and whereas these are slightly greater than one due to atmosphere in indoor. Accordingly, wave propagation get affected.

Hi

Free space means without any gas considered along the radio path in other words "free space"=vacuum.

Then if you want to consider influences of atmosphere over your radio path you need to add the attenuation given by gas, rains, snow, water vapor...etc.

All information are available in the ITU recommendations

I have selected some of them below

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.341-5-199910-I!!PDF-E.pdf

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.525-2-199408-I!!PDF-E.pdf

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.676-10-201309-I!!PDF-E.pdf

Radio planners consider a standard atmosphere to dimension radio path. See :

                 http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.835-5-201202-I!!PDF-E.pdf

All ITU recommendations related to propagation (P Series) are available here

                http://www.itu.int/rec/R-REC-P/en

Hope those documents will clarify

Best Regards

Friis formula is a nice approximation that sets the upper limit of a radio link. Very much like Shannon limit sets the upper limit of an information channel. It assumes performance when everything is perfect. Everything on top of that is akin to a perpetuum mobile.

To become more realistic, you need to put also all the losses and proximity of Earth surface into account, and you get less and less, and you need to add also some uncertainty. Because of uncertainty and various approaches to it, we have so many propagation models out there, just like Xavier nicely collected some.

If you are new to propagation, you may get a free ride with a  freeware solution at http://www.cplus.org/rmw/english1.html

There are both online and offline versions of RMW, and it is well tested and reliable. It uses public domain cartography so it is ready to go wherever you are. I even modelled WiMAX coverage of Ulan Bator with it.

Wavelength in Friss transmission formula is dependent on the speed of electromagnetic wave in that medium.

Vaccum and air have different index for transmission of EM wave and hence results in different results using friss formula.

hello tahir, according to me friss formula is generally provides the power given by  an antenna under the normal or standard condition, given by another antenna placed at some distance away from transmitting antenna.

so, the formula will remain same for the both standard conditions.

Thank you all comprehensive answers. I thought the free-space formula just valid for outdoor line of site situation. Thank you again.