# FRIIS Transmission Equation

The*Friis transmission formula*describes the fundamental antenna performance in a transmittingâ€“receiving system. This formula relates the power received by a receiving antenna to the power transmitted by a transmitting antenna provided the two antennas are separated by a distance , where d is the largest dimension of either antenna. This implies that, to apply Friis transmission formula, each antenna must be in the far field of the other.

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**Model of a transmitting and receiving system**

Here,

*P*= Time-average transmitted power

_{t}*P*

_{r}*= Time-average received power*

*G*

_{dt}*= Transmitting antenna directive gain*

*P*= Receiving antenna directive gain

_{t}