Ultra High Frequency

In terrestrial television, UHF ("Ultra High Frequency") stations are broadcasters which operate on shorter wavelengths than their VHF ("Very High Frequency") counterparts. These frequencies (nominally between 10cm and one metre) were plentiful but historically less valuable as analogue television stations on UHF channels were prone to require more power to reach even a reduced coverage area. Antenna manufacturers routinely claimed "up to 60 miles UHF, 100 miles VHF over flat terrain" for their largest rooftop antennas. UHF required different antenna designs and different tuners; in North America, many tellies made for the pre-1964 market had no UHF tuners at all.

Not all countries intermixed UHF and VHF channels in the same markets. In those which did, however, a common pattern was that the longest-established broadcasters (like NHK in Japan, CBS and NBC in the US, CBC in Canada) remained on the few available VHF channels wherever they could, leaving UHF to new entrants, fourth networks and No Budget independents.

With most prime big-city VHF stations affiliated to the strongest networks before the end of The Fifties, the rest of the spectrum became a motley array of struggling independents, small-market stations, ethnic or minority-language outlets, fourth networks and educational broadcasters. Stations on the less-desirable UHF channels (TV 14-83, 470-890 MHz in North America) operated at a substantial disadvantage in the early years and not all survived.

According to "Weird Al" Yankovic, at the time his UHF film (1989) invoked this trope: "It was a total anachronism even when it came out — it was on the tail end of UHF even being a thing. But as a kid, that was where you went to see all the weird programming. You know, you had your UHF dial, and you flipped it around, and there was everything from PBS stations to Spanish-speaking stations to low-budget public stations, to just out-and-out weirdness."

Now largely a Forgotten Trope or Dead Horse Trope, with satellite TV and digital Over-The-Air TV as Trope Breakers.

Examples of Ultra High Frequency broadcasting include:

Film

In UHF, "Weird Al" Yankovic lampshades the pattern of independent Struggling Broadcasters landing on some of the worst spots on over-the-air TV by having a fictional "Channel U-62" struggle to broadcast Cloudcuckoolander nonsense as a No Budget operation in a desperate attempt to remain on-air. Effectively, the UHF station serves as a framing device, into which to insert a long series of Show Within a Show parodies, fake ads and bogus cinema trailers.
CIVIC-TV 83 from Videodrome (1983) is a fairly obvious parody of the early CITY-TV 79 Toronto as independent No Budget Struggling Broadcaster.

Western Animation

In King of the Hill, Nancy Hicks-Gribble works for Channel 84. That's off the end of the dial (which never did go past 83) and could also be a joke on how "backwoodsy" Arlen is, as low power stations in remote areas are/were usually stuck with the worst channel locations.

Real Life

In Japan, the Otaku O'Clock pattern of programming anime in the wee hours is closely associated with small, independent UHF outlets.
In the US, educational PBS, fourth commercial network Fox and Spanish-language Univisión rely disproportionately on UHF stations. Marginal network providers like Ion Television, the infomercials' own network, tend to own and operate disproportionate numbers of outlying UHF stations. See American Television Stations and Broadcasting in the United States for the history of these.
Ted Turner's WTBS 17 Atlanta was a Struggling Broadcaster in its early days as a UHF independent; the Perpetual Poverty ended when it was uplinked to satellite as a 'superstation' in the mid-1970s.
Alas, Technology Marches On. The UK moved all of the telly channels to UHF during the transition from monochrome to colour. Many long-established North American network affiliates abandoned once-valuable low-VHF channels due to impulse noise and interference in the digital age; some of these channels are now used by no more than a dozen US full-service DTV broadcasters nationwide. As high-UHF channels are lost to mobile telephone companies, available channels for over-the-air HDTV are becoming increasingly scarce and the remaining UHF channels are valuable. The distinction in frequency has become less significant with modern equipment and with the widespread adoption of cable and satellite.