A repeater is a radio system that helps to increase the range of portable and mobile radios. VHF and UHF signals, the type used for local communications, travel roughly line of sight. As long as there are no obstructions between the radios, the signal may travel several miles.
Direct or Simplex is communication from one radio directly to another without going through any other equipment like a repeater or gateway. This works well around the neighborhood or around the ranch from radio to radio. A base radio with an antenna mounted on the roof of a building may be able to talk to radios 10 or 15 miles away but the radios in the field may not be able to hear each other.
The problem with direct communication is that buildings and other structures in an urban environment can block the signal path between radios. It may not be possible to talk more than a few blocks apart in a big city.
A repeater gets the antenna up over the buildings and trees. The mobile and portable radios have a clear line of sight to the repeater antenna. A repeater with its antenna on top of a tall building or a few hundred feet up on a broadcast tower can cover a very large area and even penetrate into buildings and down town streets. Communication through a repeater is called Duplex. In this mode, the radio will transmit on one frequency and receive on a different frequency. This is how the repeater can relay the conversation in real time. Repeaters use special filters called duplexers to allow simultaneous receiving and transmitting. The use of two different frequencies and the duplexer filter makes it possible for the repeater to receive weak signals while transmitting a strong signal and keeps the repeater transmitter from burning out its receiver.
Sometimes, repeaters are linked to other repeaters in different cities or connected to other equipment like weather stations or telephone lines. Repeaters not only increase the range of portable radios, they can add new functionality and alternative modes of communication that can be accessed on the user’s radio.