From now until July, TV broadcasters in the U. S. are in the final phases of a grand reshuffle of broadcast frequencies that has been going on for several years. Unless you happen to watch TV the old-fashioned way—by getting a signal from a rooftop or indoor VHF/UHF antenna directly from the terrestrial broadcast transmitter—you probably haven't even noticed. But this is the tail end of a process that began back in 2012, when the U. S. Federal Communications Commission (FCC) auctioned off a slather of frequencies in the 600-700 MHz range to be used as a part of the new 5G mobile-phone plan.
You may not think of the radio spectrum this way (if at all), but it is a limited natural resource, like fresh water or land. As humanity has learned how to exploit it in increasingly effective ways, the value of various parts of it has fluctuated, mostly upward, but not always. For the first seventy years or so of the FCC's existence, the agency treated the spectrum like the federal government treated federal land: if you qualified, you could just get some of it for free, and then it was yours to use or sell just like any other private property.
This wasn't always the best or the fairest way to do things. Back in the 1920s, when it wasn't clear that radio would amount to much more than some hobbyists annoying their neighbors with loud spark-gap transmitters, it seemed like a reasonable approach. But by the 1950s, when radio and then television frequencies were valued on the private market in the millions of dollars, politicians began to pull strings and the whole thing got very complicated. For example, how much of a coincidence was it that the application for a new TV station that then-Senator Lyndon B. Johnson wanted to build in Austin in the early 1950s was the only application filed in that city? None at all, because everybody else knew that LBJ was so connected in Washington that filing a competing application would be a waste of time. So LBJ's family became the proud owners of the first TV station in Austin in 1952, and the next TV station there didn't open until 1965.
Eventually, laws were passed so that the FCC could actually hold auctions to allocate new spectrum frequencies. This change acknowledged that the radio spectrum had value, and probably a better way to allocate it than political influence was to sell it to the highest bidder.
And of course, technology wasn't standing still during this time, either. When the first UHF TV band was opened in 1952, it was viewed as the most wasted part of the "vast wasteland" of TV, in the words of a cynical FCC commissioner. Originally it covered the entire frequency range from 470 MHz to 890 MHz, with channel numbers designated 14 through 83. Because a TV channel then occupied about 6 MHz, in principle there was room for almost 70 channels in the UHF band. But for many years, that promise went largely unfulfilled for technical reasons.
It was a considerable challenge to early consumer-TV makers to build a UHF tuner, which is the "front-end" part of the TV that takes the signal from the antenna and converts it down to a reasonably low frequency to be demodulated and used. Those old UHF tuners were fussy, handmade devices that you tuned with a continuously-rotating knob, like a radio dial. And they were very subject to interference from other UHF stations. Because of these problems, the FCC handed out a whole lot fewer UHF frequencies than it looked like at first glance you could fit in that huge range, because if the spectrum got anywhere close to crowded, all the UHF tuners would start picking up the wrong signals and everything would go to pot. Also, UHF signals didn't carry as far as the lower VHF frequencies (channels 2-13), so a lot of early UHF stations were local low-budget affairs that couldn't afford anything better.
Technical times changed, as they always do, and around 2000 the TV industry made its move to digital broadcasting. This change, plus advances in tuner design, rendered the old super-cautious FCC allocations pointless. And with the advent of cable TV, the importance of over-the-air broadcasting began to wane, and once tuning your TV became a job for a computer, the channel numbers no longer had to be irrevocably fixed to particular frequencies, as they had to be with electromechanical tuners.
Fast-forward to 2012. The new 5G mobile phone service plan includes the use of a 600-700 MHz band that will allow base-quality service over a much wider area than the current higher-frequency mobile phone cells permit. The problem was, there were still a lot of TV stations in that frequency range, hanging on to their old UHF TV allocations. The FCC made them a deal: if you let us auction off your frequency for 5G, we'll either share some of the profits with you and you can take the money and go off the air, or move to another frequency. Either way, we've got to clear this band for 5G. Kind of a spectrum-allocation eminent-domain action, as it were. Some stations took the money and quit. Others have been shifting up and down the frequency spectrum in a ten-phase process that will be completed by July of 2020. While this can be a big deal for the broadcasters, involving costly new transmitters and transmitting antennas, the most that even off-the-air consumers will notice is that a station may go blank, but all you have to do is "rescan" your digital TV, and it will automatically hunt for the new frequency and find it for you.
To a geezer like me, who grew up having to get up off the chair and twiddle with the fine-tune control on the TV tuner every so often, it all seems too easy. And there's something odd about the fluid shifting going on behind the scenes. Back when a channel allocation was something to be proud of, stations often incorporated their channel number in their logo. For example, in Fort Worth, the local independent station was Channel 11, and their logo featured the two numeral 1's as two nattily-dressed guys in little white suits, complete with handkerchiefs in their breast pockets (I may be imagining the handkerchief part, but you get the idea).
No longer. It's all as invisible as sewer pipes now, and about as interesting to the average consumer. But in case you were wondering where your off-the-air station went, this may be part of the explanation.
Sources: Not being a watcher of TV any longer myself, I learned about this process from an article in the San Jose Mercury-News at https://www.mercurynews.com/2020/04/30/heres-why-you-might-need-to-rescan-your-tv-right-now/. I also referred to articles from Gizmodo at https://gizmodo.com/5g-is-forcing-hundreds-of-tv-channels-to-change-how-the-1837111135and Venturebeat at https://venturebeat.com/2019/12/10/the-definitive-guide-to-5g-low-mid-and-high-band-speeds/
and the Wikipedia article on UHF TV broadcasting. The FCC has a handy map on which you can look up your local TV stations and see what's going on with their channel moves, if any, at https://www.fcc.gov/media/engineering/dtvmaps. And I got the short version of the KLBJ story from Slate, which summarizes LBJ biographer Robert Caro's extensive research on the matter at https://slate.com/news-and-politics/2007/07/how-lady-bird-and-lyndon-baines-johnson-came-by-their-millions.html.