Appendix

Relationship between Gain and Effective Apperture of an Antenna

Let us consider two antennas with maximum effective areas A_1m and A_2m and directivities (maximum of directivity function) D₁ and D₂. Let the antennas be separated by a distance R. If antenna 1 is used as transmitting antenna and antenna 2 as receiving antenna; the power received by the antenna 2 and delivered to the load will be

or     

If antenna 2 is used as transmitter and antenna 1 as receiver, for a linear isotropic medium,