Here’s how it worked.
Prior to 1996, all television signals were analog (NTSC). Most cities had a fully populated VHF band (2-13) and a thinly populated UHF band (14-83). In the 1980s, channels 70-83 were reallocated to mobile frequencies, and UHF stations in that band were reassigned.
After 1996, the FCC gave a second channel to existing TV stations so that they might broadcast digitally. The digital tuners could be programmed to keep track of frequencies–so “Channel 9” might actually be broadcasting on channel 35. Most unassigned channels were in the UHF band, so these secondary digital channels were assigned UHF licences. Thus, ordinary UHF antennas were sold to a somewhat gullible public as “HDTV” antennas.
After analog broadasting ceased in 2009, these old NTSC frequencies were no longer needed, and these digital stations could be migrated back to the old channel slots, if desirable.
And because the tuner kept track of what frequency a “tv channel” was actually using, and because digital television doesn’t need guard bands, the whole block of uhf frequencies can be squeezed down again. Channels 51-69 have been sold off to cell phone companies.
