Marine radio was the first commercial application of radio technology, allowing ships to keep in touch with shore and other ships, and send out a distress call for rescue in case of emergency. Guglielmo Marconi invented radio communication in the 1890s, and the Marconi Company installed wireless telegraphy stations on ships beginning around 1900. Marconi built a string of shore stations and in 1904 established the first Morse code distress call, the letters CQD, used until 1906 when SOS was agreed on. The first significant marine rescue due to radio was the 1909 sinking of the luxury liner RMS Republic, in which 1,500 lives were saved. This and the 1912 RMS Titanic rescue brought the field of marine radio to public consciousness, and marine radio operators were regarded as heroes. By 1920, the US had a string of 12 coastal stations stretched along the Atlantic seaboard from Bar Harbor, Maine to Cape May, New Jersey.

The first marine radio transmitters used longwave bands. During World War I amplitude modulation was developed, and in the 1920s spark radiotelegraphy equipment was replaced by vacuum tube radiotelephony allowing voice communication. Also in the 1920s, the ionospheric skip or skywave phenomenon was discovered, which allowed lower power vacuum tube transmitters operating in the shortwave bands to communicate at long distances.