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Thus, they can quickly disappear into space. Although this means that FM stations don’t interfere with each other, this characteristic creates a couple of other problems. Because of their higher frequency ionospheric refraction doesn’t appreciably affect FM or TV signals. But, unlike AM radio stations, FM stations don’t end up being assigned frequencies with nice round numbers like 820 or 1240. FM stations must be 200kHz am fm radio headphones apart at these frequencies, which means that there’s room for 200 FM stations on the FM band. This may explain why your favorite AM radio station goes off the air at sunset, or becomes much harder to hear. The ionosphere is much more effective in reflecting these radio waves at night.