Postwar American Television
In the early 50s the FCC allocated channels 14-83 (470-890 mHz) for TV broadcasting. No sets had tuners that could cover those freqencies at the time, so external converters were sold. Thes devices did not have RF amplifiers, so their noise figure was terrible, and unless you lived near a UHF station they didn't work very well. Later, the FCC required all TV sets to be made with UHF tuners, but they generally didn't work very well, making reception of UHF channels difficult.