An amateur radio repeater is an electronic device that receives a weak or low-level amateur radio signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. Many repeaters are located on hilltops or on tall buildings as the higher location increases their coverage area, sometimes referred to as the radio horizon, or "footprint". Repeaters are not limited to amateur radio (ham radio), they are in use by a wide range of users - public safety (police, fire, etc.) business, government, military, and more.
In amateur radio, repeaters are typically maintained by individual hobbyists or local groups of amateur radio operators. Many repeaters are provided openly to other amateur radio operators and typically not used as a remote base station by a single user or group. In some areas multiple repeaters are linked together to form a wide-coverage network, such as the linked system provided by the Independent Repeater Association which covers most of western Michigan, or the Western Intertie Network System ("WINsystem") that covers most of California .
In many communities, the repeater has become the on-the-air gathering spot for the local amateur radio community. Local public service nets may be heard on these systems and many are employed by weather spotters. In an emergency or a disaster a repeater can sometimes help to provide needed communications between areas that could not otherwise communicate. Until cellular telephones became popular, it was common for community repeaters to have "drive time" monitoring stations so that mobile amateurs could call in traffic accidents via the repeater to the monitoring station who could relay it to the local police agencies via telephone.
In order to get better receive coverage over a wide area, a similar linked setup can also be done with what is known as a voted receiver system. In a voted receiver, there are several repeater nodes setup to receive on the same frequency. The repeater node with the strongest signal will be the one that actually triggers the central repeater transmitter to begin transmitting with its signal. Such a system can be used to widen coverage to low power amateur transmitters that would not be able to key up the central location, but can receive the signal from the central location without an issue.
Repeaters may also be connected to over the Internet using voice over IP (VoIP) techniques. VoIP links are a convenient way to connecting distant repeaters that would otherwise be unreachable by VHF/UHF radio propagation. Popular VoIP amateur radio network protocols include Echolink, IRLP, WIRES and eQSO.
The most basic repeater consists of an FM receiver on one frequency and an FM transmitter on another frequency usually in the same radio band, connected together so that when the receiver picks up a signal, the transmitter is keyed and rebroadcasts whatever is heard.
Ham repeaters are found mainly in the VHF two meter (144 - 148 MHz) and the UHF 70 centimeter (420 - 450 MHz) bands, but can be used on almost any frequency pair above 29 MHz. Note that different countries have different rules; for example, in the United States, the two meter band is 144-148MHz, while in the United Kingdom and most of Europe) it's 144-146MHz.
Repeater frequency sets are known as "repeater pairs," and in the ham radio community most follow ad hoc standards for the difference between the two frequencies, commonly called the offset. In the USA two-meter band, the standard offset is 600 kHz (0.6 MHz), but some non-conforming oddball-split repeaters can be found in various places, and for various reasons. The actual frequency pair used is assigned by a local frequency coordinating council.
In the days of crystal-controlled radios, these pairs were identified by the last portion of the transmit (Input) frequency followed by the last portion of the receive (Output) frequency that the ham would put into the radio. Thus "three-four nine-four" (34/94) meant that hams would transmit on 146.34MHz and listen on 146.94MHz (while the repeater would do the opposite, listening on 146.34 and transmitting on 146.94). In areas with many repeaters, "reverse splits" were common (i.e., 94/34), to prevent interference between systems.
Since the late 1970s, the use of synthesized, microprocessor-controlled radios, and widespread adoption of standard frequency splits have changed the way repeater pairs are described. In 1980, a ham might have been told that a repeater was on "22/82" -- today they will most often be told "682 down." The 6 refers to the last digit of 146MHz, so that the display will read "146.82" (the output frequency), and the radio is set to transmit "down" 600kHz on 146.22MHz.
Repeaters typically have a timer to cut off retransmission of a signal that goes too long. Repeaters operated by groups with an emphasis on emergency communications often limit each transmission to 30 seconds, while others may allow three minutes or even longer. The time restarts after a short pause following each transmission, and many systems feature a beep or chirp tone to signal that this has taken place.
Most repeaters are remotely controlled through the use of audio tones on a control channel.
Repeaters can be setup as a "Link System" where transmitting on one repeater simultaneously transmits on all repeaters in the system. These systems are used for area or regional communications, for example in Skywarn.
SSTV repeaters are used by amateur radio operators for exchanging pictures. If two stations can not copy each other, they can still communicate through a repeater.
To activate a repeater the station must send a tone of frequency 1750 Hz. Then the repeater is activated and sends K in morse code. The station must start sending a picture in approximately 10 seconds. After reception the received image is transmitted on the repeater's operation frequency.
Repeaters should operate in common SSTV modes, but it depends on the software used (MMSSTV, JVComm32, MSCAN). Some repeater are not activated by audio tone, but instead by the SSTV vertical synchronization signal (VIS code).
When there is no activity on the repeater's frequency, it works as a beacon and periodically send a random picture with identification and a timestamp.
The term "Kerchunk" can also apply to the sound a large Amplitude Modulation Transmitter makes when the operator switches it off and on.