Many years ago it was decided that civil aircraft communications radios would use the118-137 MHz band, and would use amplitude modulation ("AM"). Like many otherthings in aviation, this has remained unchanged for many decades. It's a pity, because ourair-ground communications would have much better audio quality if we could switch tofrequency modulation ("FM") as the majority of commercial broadcast stationshave. But it looks like we're stuck with AM for the foreseeable future.
This is a companion discussion topic for the original entry at https://www.avweb.com/avionics/vhf-nav-comm-basics