From The RadioReference Wiki

Repeater type radio systems are a special kind of duplex radio system, where a repeater station listens on one frequency (the Input frequency) and simultaneously retransmits the signal it receives on another frequency (the Output frequency). All of the mobiles and base stations listen on the repeater output frequency and transmit on its input frequency. The transmit/receive frequency difference is the Offset. This type of operation solves many of the problems inherent to the duplex type of operation, while clearly being superior to simplex operation.

Another advantage of this type of operation is that the repeaters are often placed at remote sites to take advantage of terrain. This allows the extending of communication coverage area by taking advantage of items that used to cause problems, such as mountainous terrain. Remote sites that are all part of one system can be connected so that radio traffic heard at one site will be retransmitted on the other sites. This connection can be by landline, UHF links, or microwave. If the repeater is on top of the mountain, then line-of-site communication is greatly extended as the line of site is between the repeater and mobile stations on either side of the mountain, thus allowing the mobiles to easily communicate. It should be noted that Trunking Communications utilizes a series of repeaters in a special way.

In the RR Database, a repeater is designated by the letter "R" in the "Type" column.

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