A UHF repeater works on 70 cm (420 to 450 MHz in the US), 33 cm (902 to 928 MHz) or 23 cm (1240 to 1300 MHz). On 70 cm, repeaters are everywhere; 33 and 23 cm have far fewer. The rules are the same as for VHF (a Technician or higher may hold the station), but the bands are shared with other services, so there are more conditions to read before you choose a site.
Why build on UHF
Buildings. UHF wavelengths of a foot or two pass through windows and gaps and reflect around inside, so a handheld indoors or in a city canyon often works better than on VHF. Over open country UHF usually reaches a little less.
Small antennas. A UHF antenna with real gain is compact, and a handheld's whip is short. The cost is cable: feed-line loss rises with frequency, so UHF sites use shorter runs of larger, low-loss cable; see Feed line loss and velocity factor.
Easier duplexers. A bigger offset, as a share of the frequency, means the transmit and receive filters need not be as sharp. See Repeater hardware and duplexers.
Room in cities. Where 2 m is crowded, 70 cm offers more channels. In the ARRL plan, 70 cm repeater blocks are left as a local option and the usual pairing is 5 MHz, which your coordinator confirms.
Illustrative pairs: 146.94 MHz, 447 MHz, 927.5 MHz and 1285 MHz outputs. Free-space quarter wave; a real whip is cut a little shorter.
Where repeaters can go
Three layers decide, and the diagram shows them on 70 cm. The rule (47 CFR 97.205(b)) bars repeaters from 431 to 433 MHz and 435 to 438 MHz. The band plan is a voluntary guide with repeater, link and satellite segments. And sharing says who else is there.
Simplified from 47 CFR 97.205 and 97.303 and the ARRL band plan. Local plans differ; read the rules.
Sharing the band
Amateur radio is a secondary user of these bands. Part 97.303 says amateurs on 70 cm, 33 cm and 23 cm must not cause harmful interference to, and must accept interference from, US government radar (radiolocation). The general lesson: you do not own the band, and radar and other government users can degrade your repeater.
Some limits are geographic, so read them before choosing a site:
70 cm: no transmitting north of Line A in 420 to 430 MHz, and extra limits near Buffalo, Cleveland and Detroit.
33 cm: no transmitting from part of Texas and New Mexico, and some segments are barred in parts of Colorado and Wyoming. Receivers there must also accept interference from industrial, scientific and medical equipment.
23 cm: shared with aeronautical navigation, satellite navigation and space-research services.
Rules are updated from time to time, so check the current text of 97.303, and ask your coordinator.
23 cm: links and mesh
23 cm is thinly populated with repeaters. Its ARRL plan includes FM and digital repeater inputs at 1270 to 1276 MHz paired with outputs at 1282 to 1288 MHz, point-to-point links, and ATV channels. Its chief use is links: an auxiliary station (97.201) may use any amateur band from 2 m up except a few segments, so a repeater system can join sites on 23 cm, away from the busy channels. Wideband data and mesh networks tend to use the microwave bands, and some use 33 cm; see AREDN and mesh networks. Outside the US, band edges and splits differ.