"James Robinson" wrote in message

With AM, when two radios transmit on close frequencies, you either hear
both signals poorly, or you get squeal, which is the sum of the two
signals. This characteristic is considered important when you have
elevated
transmitters that can be hundreds of miles away, like on aircraft.

Sounds plausible. Marine radios also operate in the VHF band, but are FM.
They are also almost always at or very close to sea level.