TRAIL Spotlight: Ionospheric Radio Propagation

How can our radios receive signals from stations beyond our line of sight? One common mode of transmission and reception of radio waves is “skywave,” or ionospheric propagation. In skywave transmission, a radio station transmits medium or high frequency (MF or HF) radio waves away from the Earth and toward the ionosphere, an atmospheric layer extending from approximately 50-400 miles (approximately 80-640 km) above the ground. The ionosphere gets its name from the high concentrations of charged ions and free electrons present at this altitude. These charged particles act as a mirror for the transmitted radio waves, reflecting them back toward Earth. The skywave can bounce once, or many times, between the ionosphere and the Earth, before reception by a radio receiver. We experience skywave reception with long-distance AM radio reception, as well as for shortwave radio and many of the radio bands used by amateur radio “hams.”