Figure 8.7a shows the power transmission from a transmitter (transmitted power PTX, antenna gain GTX) to a receiver (received power PRX, antenna gain GRX). 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
We reformulate the Friis equation by using logarithmic representation of the quantities.
where LF0 is the free space loss.
The free space loss in decibels (dB) gives us the ratio of transmitted power PTXi and transmitted power PRXi in the case of isotropic radiators (see Figure 8.7b).