A VHF repeater works on 6 m (50 to 54 MHz), 2 m (144 to 148 MHz) or 1.25 m (222 to 225 MHz). VHF is the classic repeater territory: signals travel mostly in straight lines, so antenna height buys range, and a hilltop site lets a handheld reach across a region. A Technician or higher licensee may hold a repeater station and be its control operator; Technicians have full privileges on all three bands (47 CFR 97.205(a), 97.301).
What VHF is good at
Coverage from height. The radio horizon is a little beyond the visible one (see Line of sight and the radio horizon), so a tall site matters more than extra power. Lower VHF also bends over hill crests slightly better than UHF.
Manageable hardware. Antennas are a few feet long, and cable loss is modest, so a good antenna at the top of a tower is practical. See Repeater antennas and coverage.
FM that ignores static-type noise, which suits mobile and handheld users; see FM voice operation.
The trade-off is how far VHF sometimes travels. Two quirks create co-channel interference between repeaters on the same pair:
Tropospheric ducting. A temperature inversion can trap VHF near the ground for hundreds of miles, so a distant repeater's output suddenly arrives on your channel. See Tropospheric ducting and scatter.
Sporadic E on 6 m (and sometimes 2 m), most often in late spring and summer, can bring in stations and repeaters from far away; see Sporadic E.
Coordinators space repeaters that share a pair by distance, and an access tone (CTCSS) keeps far-off signals from keying yours, but neither removes the problem entirely.
Pairs and sub-bands
Part 97 keeps repeaters out of the weak-signal and satellite segments (50.0 to 51.0 MHz, 144.0 to 144.5, 145.5 to 146.0 and 222.00 to 222.15 MHz). Inside the rest, the ARRL band plan, which is voluntary, lays out blocks for repeater inputs and outputs, and a local frequency coordinator recommends the actual pair; see Frequency coordination and the ARRL. Slide the output below to see its input.
Minus offset low in the band, plus offset from about 147 MHz. Coordinators decide the actual pairs; check the current band plan.
Offset−600 kHz
On 2 m the direction follows the part of the band: outputs from 147.00 MHz up usually use a plus offset, the others a minus. The plan itself notes that locally coordinated plans take precedence over the national one, and that some regions use channels differently, so treat the picture as the national default and your directory as the truth.
In practice
Power and height. Transmitters typically run tens of watts at the antenna. More power rarely helps, because the weak handheld signal going back to the repeater limits range, not the repeater's strong output. Height is the biggest lever.
The duplexer. A 600 kHz split is a tiny fraction of 146 MHz, so the filters must be sharp, which usually means large high-Q cavities. This is often the bulkiest part of a 2 m site; see Repeater hardware and duplexers.
Rules that matter. A repeater may be automatically controlled, may limit access to chosen users, and is not accountable for rule-breaking transmissions it retransmits by accident (97.205(d), (e), (g)). If two repeaters interfere, both licensees are responsible, unless only one is coordinated (97.205(c)). See FCC rules for repeaters.
Other countries differ. In Europe and Africa 2 m is 144 to 146 MHz, and 1.25 m is largely an Americas band, so offsets and band plans vary by country.