I do not understand a basic concept about Friis transmision equation between two antennas:
where Dt and Dr are the antenna directivities (with respect to an isotropic radiator) of the transmitting and receiving antennas respectively, λ is the wavelength and d is the distance between the antennas.
Let's consider for instance the case in which both are dipole antennas, whose pattern is drawn below:
How can I apply the previous formula to this case? Precisely, I have the following doubt:
1) Since dipoles are not isotropic radiators/receivers, their directivities will depend on the direction considered. So, I'd say that Pr will depend on direction. How do I find the total received power?
2) Part of the radiation from TX goes at also other directions (for instance at left). Is this fact considered in the Friis equation (and, if yes, which factor describes it?)? How can I distinguish, numerically, a situation like that in the previous picture, from something like this (a RX antenna, drawn in green, that surrounds the TX and so receives more power):