10-29-2012, 03:06 PM
There's still some AM used on VHF, albeit not hi-fi. Namely the aviation band just above 108MHz.
It would be difficult or expensive to build and sell broadcast AM radios for the VHF band, the front ends would likely drift too much to stay on a 20KHz wide AM station. You see this problem on higher SW band radios trying to tune in a station around 19MHz. The radios that convert immediately from 19MHz to the 455KHz IF, the local oscillator, usually a pentagrid converter using an LC circuit drifts a lot. Oh, crystals would work better, but are too expensive. FM broadcast stations are much wider, about 200KHz, so drift is less serious to keep one station tuned in.
It would be difficult or expensive to build and sell broadcast AM radios for the VHF band, the front ends would likely drift too much to stay on a 20KHz wide AM station. You see this problem on higher SW band radios trying to tune in a station around 19MHz. The radios that convert immediately from 19MHz to the 455KHz IF, the local oscillator, usually a pentagrid converter using an LC circuit drifts a lot. Oh, crystals would work better, but are too expensive. FM broadcast stations are much wider, about 200KHz, so drift is less serious to keep one station tuned in.
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