The 1930s witnessed a technological war between the US and Mexico. Radio channels overlapped with each other, turning the nighttime radio world into bilingual chaos. How did it end? Physics! And diplomacy.
After decades of fighting, Canada, the United States, and Mexico got together to establish certain radio signals throughout the western hemisphere. They needed a little help from physics. They isolated which frequencies could maintain a skywave, and they called those frequencies "clear channel stations." The stations assigned to these frequencies would be required to maintain a certain level of power at all times, and lower-powered stations couldn't move in on these stations and disrupt a larger broadcast. Local, low-power stations were also assigned a certain set of frequencies, so their broadcasts wouldn't be overpowered by national stations.