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pseydtonne · 4171 days ago · link · · parent · post: A Quest to Save AM Before It’s Lost in the Static - NYTimes.com
It's not the stronger signal that makes AM carry farther. There are plenty of 50kw FM stations and AM stations: same power, different range. It has to do with Earth's atmosphere. The ionosphere acts like a pool of water high in the sky: frequencies lower than 30 MHz bounce back to Earth, while those above go through. Thus if we had FM stations at 20 MHz, we'd probably be able to hear them over longer distances. The result for AM is that it goes up, bounces down -- and bounces even more over bodies of water. The old definition of a clear channel station (before that became a brand) was an AM station audible up to 750 miles away at night.