Repeater station
From AB9RF
A repeater station (or simply a repeater) is a radio station whose purpose is to extend the range of communications.
A basic repeater station consists of a receiver tuned to one frequency and a transmitter tuned to a different frequency. Whenever the receiver receives a signal, the repeater's controller activates the transmitter and retransmits whatever signal is being received by the receiver.
More complicated repeater stations may have multiple inputs at different locations or on different frequencies; in such repeater stations the controller will have an additional circuit called a "voter" which decides which of several active inputs is selected for retransmission. There may also be multiple transmitters, either all transmitting simultaneously or, possibly, selectively based on the current configuration selected by the repeater's control operator.
In the amateur radio service, most repeater stations operate in the 2 meter VHF and 70 centimeter UHF bands. Regulations prohibit the operation of repeaters in bands below 10 meters. Virtually all repeaters, other than those in the satellite service, use the same modulation on both input and output, the modulation being either analog narrowband FM or one of several digital voice standards (typically D*STAR, although there are a few APCO 25 repeaters as well).
If the receiver is located near the transmitter (and especially if both use the same antenna), and both operate in the same band, very tight filtering is required to isolate the signal coming in from the antenna for the receiver from the signal going out from the transmitter to the antenna. For most VHF repeaters, cavity filters (also known as "cans") are used for this. Various technologies, in addition to cavities, may be used in UHF and higher frequencies. Ten meter repeaters almost always use diversity siting (receive separate from transmit) as cavities for ten meters would be extremely large and difficult to fabricate. Repeaters that have input and output in different bands (known as crossband repeaters) do not require particularly complex filtering, and in fact this function is built into many commercially-available mobile amateur radios (such as the Yaesu FT-8900).
In order to use a repeater, the user must transmit a signal on the repeater's input frequency, and listen for responses on the repeater's output frequency. Virtually all VHF and UHF radios sold to amateur operators today are capable of such operation, and in fact are programmed with the national repeater offset plan for the country in which they are being sold so that such operation is automatic; dialing in a specific receive frequency will automatically select the appropriate corresponding transmit frequency, and the radio will automatically change frequency when switching between receive and transmit. Repeaters which do not follow the standard offset plan for their location are said to have an "odd split".
In order to reduce unwanted retransmission of noise and interfering signals, most controllers require the presence of a subaudible tone, called a PL tone or CTCSS tone, as part of the received signal before the controller will activate the transmitter. This prevents unwanted signals from being retransmitted, as unwanted signals are unlikely to include that tone.
Repeaters are often installed on high terrain features or tall buildings to take advantage of the greater range offered by such placement.
Many repeaters include added features such as the ability to temporarily link the repeater to another repeater (by a wireline link, radio link, phone line, or the Internet), to place phone calls (a function known as "autopatch"), and the ability for the repeater controller to make automatic announcements of interest to the community which uses the repeater. In many cases some or all of these capabilities can be controlled via the primary receive frequency, typically using DTMF codes.
