Microwave radio systems typically operate at frequencies above 1 GHz. Microwaves propagate mainly in the line-of-sight or free-space mode, whether in ground or satellite systems. Microwave radio systems have been used in long-distance telephone communications since the 1950s. The system provides the necessary transmission bandwidth and reliability to enable the transmission of thousands of telephone channels and several television channels over the same facility and route. It uses carrier frequencies in the 3-12 GHz range. Since microwaves travel only in line of sight, repeater stations are required at intervals of about 50 km. The transmitter output power is low (may be less than 1 watt) as highly directional high gain antennas are used in this system.
The figure below shows the equipment required to supply a channel of microwave system. It consists of two terminal stations and one or more repeater stations. At the transmitter end, several hundred telephone channels and/or one television channel are frequency multiplexed into a baseband bandwidth of 6 MHz to form the input .The base band frequency modulates a 70 MHz IF signal, which is up-converted in the 4 GHz band to the microwave output frequency f1. This signal is amplified and fed through a directional antenna to a repeater station about 50 km away. At the repeater station, a signal of frequency f) is received by an antenna pointing towards the original station. The IF signal is then down-converted. Again at the receiving terminal it is up-converted to the new frequency f2 for transmission towards the station. When the signal passes through several repeater chains, alternate links in the chain use alternate frequencies, so that the energy retransmitted to a repeater station does not feedback into its own receiver. Some microwave systems use a few two-way channel pairs, and repeater stations use more complex frequency switching systems.