Key:communication:radio:repeater

A repeater receives input, often on one or more fixed frequencies, then retransmits it on a fixed frequency. If you are using a repeater and you shift your transmit frequency the output of the repeater does not shift - and at some point the repeater receiver won't hear you. If you shift your transmit frequency somewhat (within limits) to a transponder the output of your signal from the transponder will shift as well.

Radio amateurs typically use repeaters on microwave bands in order to communicate within a region using hand-held radios.