K
K.M. Richards
Guest
While this question is somewhat tangential to the thread's intent, here is a quick answer.
In markets where VHF channel allocations were not possible because of minimum mileage separations, but geography precluded actual reception of the protected VHF allocations, markets were made all-UHF (or mostly UHF) and in those markets, since one had to have a UHF-capable TV set, the manufacturers made every effort to engineer the tuners better. In those markets, UHF was viable in the 1950s.
In other markets, UHF was considered secondary in nature and manufacturers did not include UHF tuners until forced to by federal law in 1964. Even then, sets designed for sale where VHF was dominant did not include the best quality UHF tuners, and UHF took longer to develop.
In the late 1970s, improvements to UHF tuners were mandated, and "click-tune" was required for both UHF and VHF. This began to close the gap, and the further development of non-rotary tuning systems in the 1980s brought better parity. By the late 1980s it was rare to see a UHF go dark and even failing UHF stations could often locate a buyer instead.
Also in the 1980s and 1990s, the increase in cable subscribers put UHF stations on a much more equal footing than before.
Of course, now that digital broadcasting has been proven to work better on UHF than VHF channels, the majority of stations transmit on that band, regardless of what PSIP shows as their virtual channel number.
I think you can see that there was no specific "take off" point. It was a gradual process and different parts of the country had different timelines.