500 kHz

Since early in the 20th century, the radio frequency of 500 kilohertz (500 kHz) has been an international calling and distress frequency for Morse code maritime communication. The unit kilohertz was not introduced until the 1960s. For most of its history, the international distress frequency was referred to by its equivalent wavelength, 600 meters, or, using the earlier frequency unit name, 500 kilocycles (per second) or 500 kc.

The United States Coast Guard and comparable agencies of other nations used to maintain 24-hour watches on this frequency, staffed by highly skilled radio operators. Many SOS calls and medical emergencies at sea were handled here until December 1999. However, because of the near disappearance of the commercial use of Morse code, the frequency is now rarely used. Emergency traffic on 500 kHz has been almost completely replaced by the Global Maritime Distress Safety System (GMDSS). Beginning in the late 1990s, most nations ended monitoring of transmissions on 500 kHz. The nearby frequencies of 518 kHz and 490 kHz are used for the Navtex component of GMDSS. Proposals to allocate frequencies at or near 500 kHz to amateur radio use resulted in the creation of the 600-meter amateur radio band.

International standards for the use of 500 kHz first appeared in the second Berlin International Radiotelegraphic Convention, which was signed November 3, 1906 and became effective July 1, 1908. The second Service Regulation affixed to this Convention designated 500 kHz as one of the standard frequencies to be employed by shore stations, specifying that "Two wave lengths, one of 300 meters [1 Mc/s] and the other of 600 meters, are authorized for general public service. Every coastal station opened to such service shall use one or the other of these two wave lengths." (These regulations also specified that ship stations normally used 1 MHz).

International standards for the use of 500 kHz were expanded by the Third International Radiotelegraphic Convention, which was held after the sinking of the RMS Titanic. This Convention, meeting in London, produced an agreement which was signed on July 5, 1912, and became effective July 1, 1913.

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