Relay station

A broadcast relay station, satellite station, relay transmitter, broadcast translator (U.S.), rebroadcaster (Canada) or repeater (two-way radio), Complementary station (Mexico), is a broadcast transmitter which repeats or transponds the signal of another radio station or television station, usually to an area not covered by the signal of the originating station. They may serve, for example, to expand the broadcast range of a television or radio station beyond the primary signal's coverage area, or to improve service in a part of the main coverage area which receives a poor signal due to geographic constraints. These transmitters may be (but are not usually) used to create a single-frequency network. They may also be used by a radio station on either AM or FM to establish a presence on the other band. Less commonly, a rebroadcaster may be owned by a community group rather than the owner of the primary station. An example is WHLS/WHLX of Port Huron, MI which purchased a translator, and shortly after that switched to an Alternative Rock format only mentioning their FM translator (except for their legal top of the hour ID). Neither of the AM frequencies is mentioned.

In its simplest form, a broadcast translator is a facility created to receive a terrestrial broadcast station over-the-air on one frequency and rebroadcast the same or substantially identical signal on another frequency. These stations are used in television and radio to cover areas (such as valleys or rural villages) not adequately covered by a station's main signal. They can also be used to expand market coverage by duplicating programming on one band to another.

Relays which broadcast within or very near the parent station's coverage area (a "fill-in") on the same channel or frequency are called "booster" stations in the U.S. However, this can be tricky because it is possible to have both stations interfering with each other unless they are carefully designed. Radio interference can be avoided by using exact atomic time obtained from GPS satellites to perfectly synchronise co-channel stations, as in a single-frequency network.

...
Wikipedia