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 1960. 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 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 the late 1980s. 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.