Scott Dorsey wrote:

They are heavy. I don't think it would be all that hard to convert them
to the new channel spacing (using a half-channel switch and reducing the
bandwidth), but I don't think anybody would want to, seeing how cheap
the modern UHF stuff is. And the military guys everywhere are trying to
get away from AM even for local ops stuff.
--scott

Well made points. The only add'l comment I'll make is that aviation
comms, those which will, at one point or another, be used in the global
ATC system will remain for the foreseeable future on VHF/UHF AM, albeit
considerable data is now passed via other modes/frequencies. The
airborne fleets are simply too big to convert, en mass, not to mention
the conversion of all ground-based assets. This subject has been
discussed at ICAO for a long time now and few appear interested in the
capital investment. I have a collection of some minutes and proceedings
from old RTCA meeting and symposiums in the 50's that present the idea.
Nothing is forever, though........... Finally, UHF has (like VHF did
a some time ago) gone from 100kHz spacing to the now standard 25kHz
spacing, or quadruple the number of available channels. Some have
discussed 12.5kHz spacing at meetings. The 12.5 spacing can present
some real difficulties with legacy equipment, though.
de K3HVG