Shortwaves do not follow the earth’s surface. Rather, they are aimed at the ionosphere (a layer of air which has been electrically charged, or “ionized”) with special antennas to bounce signals from one point to another, effectively enabling worldwide coverage 24 hours a day. The ionosphere “bends” radio wave beams which enter it and sends them back to earth. A major factor in shortwave broadcasting is the constantly changing ionosphere. It changes height and density, sometimes in a matter of hours.
Medium wave is the standard AM. It generally follows the earth’s surface until the signal grows so weak with distance that it is no longer receivable. How far the signal travels depends upon the frequency, the transmitter’s power and whether the route is over water or land. The performance of AM is predictable, but the distance to which the signal can reach is rather limited by time of day: short distances during the day and long distances at night.
FM is local line-of-sight broadcasting only–-the higher the tower, the better the coverage. Due to its limitations, it is not suitable for long-range broadcasting.