To understand some of the concepts of radio, we have to look at some of the strange things that happen in broadcasting and take a simplified look at how radio works.
First, let's look at the AM radio, which stands for "amplitude modulation" and it's band or group of frequencies. AM radio ranges from 535 kilohertz to 1705 Hz. These are the numbers you see on your AM radio dial. Stations can theoretically be placed every 10 kHz, along the AM band. This means that there are a total of 117 different channels available for AM radio stations.
If it all stopped there, thing would be rather simple; but, unfortunately, a lot of other factors come into play.
First, you can't put stations on the same frequency that are too close together in a geographic area or they will interfere with each other. And for the same reason, you can't have two stations close together in frequency. These are things that limit the number of radio stations in an area.
The good news is that since the signals of stations tend to be limited in their range, you can use some of the frequencies many times--as long as the stations are far enough apart geographically. This is why we can have nearly 5,000 AM stations on only 117 different frequencies.
How far an AM station's signal travels depends on such things as the station's frequency, the power of the transmitter in watts, the nature of the transmitting antenna and the ionosphere layer of molecules above the earth's atmosphere.
This "ionosphere refraction" is a big issue, since AM radio waves can end up hundreds or thousands of miles away from point of origin...and in the process, interfere with all other stations on the same frequency. The ionosphere is much more effective in reflecting these radio waves at night. That's why at sunset most AM stations in the U.S. have to: reduce power, send their signal in different directions, or even go off the air until sunrise the next day.
FM (frequency modulated) radio waves don't act in the same way as AM radio waves. For starters, they are on a higher frequency with the FM radio band going from 88 to 108 Mhz. Since stations must be 200 KHz apart at these frequencies, there's room for 200 FM stations on the FM band.
Because of their higher frequency, the ionosphere refraction doesn't appreciably affect the FM signals. For the most part, FM signals are considered "line-of-sight." They travel in a straight line and don't bend around the earth as AM ground signals do. They quickly disappear into space.
So, the farther away from the FM station you are, the higher you have to have an antenna to receive the signal. And, since the earth is round, these signals literally leave the earth after about 90 miles! The line-of-sight signals can be stopped by mountains and buildings, which cause the "swishing sound" when you listen to FM radio while driving around tall structures.