Figure 8.7a shows the power transmission from a transmitter (transmitted power *P*_{TX}, antenna gain *G*_{TX}) to a receiver (received power *P*_{RX}, antenna gain *G*_{RX}). The antennas are assumed to be perfectly matched and the distance *r* is large enough to consider the antennas to be in the farfield region, which is virtually always the case in practical communication links. Furthermore, the main lobes of the antennas point exactly at each other and the antennas exhibit the same polarization.

In free space there is a direct path for the electromagnetic wave from transmitter to receiver and the receiver power is given by the *Friis*-equation as

8.5

We reformulate the Friis equation by using logarithmic representation of the quantities.

8.6

where *L*_{F0} is the *free space loss*.

The free space loss in decibels (dB) gives us the ratio of transmitted power *P*_{TXi} and transmitted power *P*_{RXi} in the case of isotropic radiators (see Figure 8.7b).

8.7

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