All electromagnetic waves travel in a straight line while in a uniform medium such as the lower atmosphere. Therefore, most radio waves only have what is called a line-of-sight range. That means that if a mountain range or the curvature of Earth were in the way of the radio signal, the receiver would be out of range and would not receive the signal. This is why most broadcasting antennae are placed on tall buildings or mountains to help increase the line-of-sight range.

Waves with frequencies below about 30 Mhz are able to reflect off the charged particles in Earth’s ionosphere; this is referred to as “skip.” Instead of passing through the ionosphere and entering space as higher-frequency electromagnetic waves do, the lower frequencies on the AM band can be reflected back toward Earth to increase their range dramatically. After sunset the ionosphere’s altitude permits stations to be heard thousands of kilometers from a transmitting tower. A handful of AM stations are “clear channel” stations. That is, there is only one station in the continental United States broadcasting on that frequency. Those stations can be heard across almost the entire country without interference from other stations. Broadcast FM stations, with frequencies 88-108 megahertz penetrate the ionosphere, and so can only be heard 80–160 kilometers from the transmitter.