2. Specific features of the millimeter-wave spectrum

2.1 Free-space attenuation and atmospheric absorption

When designing an electromagnetic wave transmit/receive system, the power balance is impacted by the transmit power (P _t ), the realized gain of the transmit and receive antennas (G _rt , G _rr ) and, of course, the free-space attenuation of the waves, which is proportional to distance (r) and frequency. The theoretical power received between a transmitter and a receiver is thus given by the equation