Remember, Longwave has a very big reach.
If it were not for the AM interference, a low power transmission goes by far the furthest in the miles per watt ratio.
If a system such as DRM or a more modern equivalent such as a DAB MUX were operated on Longwave, whole countries can be covered with many channels, all on the one frequency.
It is only phase distortion that inhibits Medium wave and Shortwave AM transmissions. FM on these bands would not suffer phase distortion, but consumes too much bandwidth for one single station.
I have used Narrow FM on Shortwave and it is superb quality. But not practical, so a Digital MUX on Medium wave or Shortwave might be a way to go.
Thoughts?
David G1ZQC.