Why Friis equation works

Hello,

I have been wondering for a few years why Friis equation actually works.
Its Pr = Pt + Gt + Gr - PropLoss, and Pr will be in ideal case the power available at the output port of receiving antenna.
How is that possible when Pt is not 100% percent concentrated in direction of transmitting.
In the case of two isotrope antennas, if the first is fed wit x watts theu will be spread in all directions. So, how can you in that case receive Pt - PropLoss because this means that all poweer is concentrated to the receiving antenna.

I will feel iluminated if somebody could explain me this concept

The propLoss term takes into account isotropic radiators at each end, and the terms Gt & Gr correct for the actual gain of the tranmit & receive antennas.

if you look at the first equation listed in http://en.wikipedia.org/wiki/Friis_t...ssion_equation you see that they write proploss as (Lambda/(4*pi*R))^2, where R is the distance between the two antennas. This is the part of the equation that deals with the fact that energy is spreading in all directions (and specifically is decreasing by a factor of 1/R^2)

Yes, thak you. I feel much better now I must say.