What's a repeater?
A repeater is a term for a device used often in Ham Radio as well as police/emergency radio communications. It consists of typically a transceiver (a fancy term for a sometimes-glorified two-way radio, from a shortened transmitter+receiver), a controller (the "brains", which sees if someone's talking and fires up the repeater access, plus often more)and a duplexer (usually an ugly can that keeps the repeater form deafening itself with its own transmissions; see the node for it for more info.)
A repeater receives signals (normally narrow-band voice) and rebroadcasts them on a different frequency (known as the repeater shift). A repeater can be used for long(er)-distance communication on higher frequencies, mostly on 2 meter AKA 135-170 MHz bands and above. However, repeaters are occasionally known to duplex as low as 10 meters.
Why repeaters?
Well, say you want to chat with a friend on another side of a hill. Your transceiver can't see hers and vice-versa. So what to do? Why not put a repeater on top of the hill? Both of you can now talk to each other, and often many other
hams still further away (depending on the height of the hill upon which the repeater sits).
A little more on repeater shift
Why not transmit on the same
frequency than the frequency you get the signal on? It seems like it would make sense, until you think of a frequency chunk like a wire: you rarely have an amplified signal on a power amp for a stereo coming out of the "in" wire. If you do, you'd get all kinds of nasty problems with feedback, cancellation,
standing waves and other unpleasant
things. It is, however, possible sometimes to have two FM signals on top of each other with some intelligibility. However, the practice is not recommended as they are never (almost) in
phase with each other and sound terrible.
The repeater shift amount varies from band to band, and on some ham bands the sections of the band used for repeaters are paired, or overlaid on top of each other, with some overlap. What does this mean? The output frequency for one repeater with an input frequency that is shifted up sits next to a bunch of positively shifted inputs, and vice-versa.
A repeater is assigned a frequency by a regional committee known as the frequency coordinator.
Below is a sample chart for how repeaters are spaced out on the 2-meter band (a band is just a chunk of radio space). The output is shown in the boxes, and the direction and amount (in KHz) of shift for the input frequency. For example if a repeater sits on 145.435 MHz it should have an input frequency of 144.835MHz.
____ ____ _________ ____
|-600| |+600| |-600|+600| |-600|
-------------------------------------------------------
145.1-145.5 146.0-146.4 146.6-147-147.4 147.6-148
The chart was copied out of a Yaesu HT manual, I should probably improve on it, as it's rather terse
Sources:
ARRL 1994 Handbook, ©1993 the Amateur Radio Relay League
McKibbin, Mark (editor); The WWARA's Pacific Northwest Repeater Directory