A radio repeater may be a combination of a receiving set and a transmitter that receives a sign and retransmits it, in order that two-way radio signals can cover longer distances. A repeater sited at a high elevation can allow two mobile stations, otherwise out of line-of-sight propagation range of every other, to speak .Repeaters are found in professional, commercial, and government mobile radio systems and also in amateur radio.
Repeater systems use two different radio frequencies; the mobiles transmit on one frequency, and therefore the repeater station receives those transmission and transmits on a second frequency. Since the repeater must transmit at an equivalent time because the signal is being received, and should even use an equivalent antenna for both transmitting and receiving, frequency-selective filters are required to stop the receiver from being overloaded by the transmitted signal. Some repeaters use two different frequency bands to supply isolation between input and output or as a convenience.