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A repeater is an automated radio station — usually mounted on a hilltop, tower, or tall building — that receives your signal on one frequency and simultaneously retransmits it on another. Repeaters solve the fundamental limitation of two-way radio: line-of-sight range.
UHF radio signals (where GMRS operates) travel in roughly straight lines. They don't bend around hills, mountains, or buildings. Two handhelds on flat ground might reach 1-3 miles. Put a building or ridge between them and that range drops to nearly zero. A mobile radio with more power helps, but terrain is still the limiting factor.
A repeater sits at an elevated location with a clear view of a large area. When you transmit, your signal reaches the repeater even if it can't reach the person you're talking to directly. The repeater then retransmits your signal from its elevated position, covering a much wider area.
Key concept: A repeater uses two frequencies — an input (where it listens for your signal) and an output (where it retransmits). On GMRS, the input is always 5 MHz above the output. Your radio handles this split automatically when programmed with the correct offset.
GMRS repeaters use channels 15R through 22R. Each channel has a pair of frequencies separated by 5 MHz. For example, channel 15R outputs on 462.5500 MHz and takes input on 467.5500 MHz. When you program a repeater into your radio, you set the output frequency and a +5 MHz transmit offset. Your radio then automatically transmits 5 MHz higher than what you're listening to. See Repeater Offsets for the full channel pair table.
Most repeaters require a CTCSS (PL) tone or DCS code to activate. This prevents the repeater from being triggered by stray signals or interference. You'll need to program the correct tone into your radio — without it, the repeater hears you but won't retransmit. Check repeater directories for the required tone. If you can hear a repeater but can't access it, see troubleshooting repeater access.
Elevation is everything. A repeater on a 1,000-foot hill with a good antenna can cover a 30-50 mile radius. The same repeater at ground level might only cover a few miles. This is why repeater operators spend significant effort finding the best possible locations — and why access to a good repeater is the single biggest range improvement you can make with GMRS.
Now that you understand the concept, learn how to actually use one: Using Repeaters covers programming, etiquette, and best practices for repeater operation.