The FM spectrum of 42 to 50 mHz was originally established for FM transmission. It was never used for AM transmission. In the late 40s, as television became a reality, there was movement to push the FM spectrum upwards to make room for television. And then there was the theory that sunspots would disrupt FM transmission, so "for its own good" FM should be repositioned. FM proponents said this would cripple the new audio technology, making more than 50 FM stations and the half million or so FM receivers obsolete. It was determined that the sunspot danger was minimal, but inspite of this, in January, 1945, the FCC moved the FM spectrum to 88 - 108 mHz. Existing stations had to buy new transmitters; radio manufacturers had to re-tool their radios, although stations were allowed to broadcast on both bands for awhile. It set the progress of FM back at least 10 years. FM revenue didn't overtake AM until the late 70s. And this was because of the increase of car radios with FM reception. Although the FCC
legislated that TV sets had to be able to receive both VHF and UHF signals, the FCC refused to force the manufacturers to include both AM and FM. What really pushed FM forward was the ruling that in major markets, the owners of AM and FM stations had to program them separately.
The fight between Edwin Armstrong, the inventor of FM, and David Sarnoff of RCA and William Paley of CBS (they were against FM as it was technically better than their AM stations, and they feared it would devalue their property), is fascinating one, narrated in Tom Lewis' authortative book "Empire of the Air - the Men Who made Radio."
