In a radio link two identical antennas are used each one performing a different role. The directivity or gain is the natural parameter of interest for the transmitting antenna. Whereas the effective area characterized the antenna in the receiving role. Power transmission loss (Friis Equation) is only valid in free space as a general belief but in this paper it is proven that this principle is valid also for radio links over perfect ground. This statement permits the exclusion of any artificial factor to achieve the eadio link power budget. This fact takes into account the lack of loasds in the natural space between both antennas. This coecepe ise very important for antenna factor measurements or calculatioap nocsp-sary for electromagnetic compatibility (EMC) activities.
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