Longwave radio frequencies are those below 500 kHz, which correspond to wavelengths longer than 600 meters. They have the property of following the curvature of the earth, making them ideal for continuous, continental communications. Unlike shortwave radio, longwave signals do not reflect nor refract using the ionosphere, so there are fewer phase-caused fadeouts.
The earliest radio transmitters, including the Alexanderson alternator, were all longwave transmitters.
In Europe, North Africa and Asia, longwave radio frequencies between 153 and 281 kHz are used for domestic and international broadcasting. In the Americas, frequencies between 200 and 430 kHz are used for non-directional radio beacons, and do not necessarily follow the same 9 kHz spacing that other areas do.
The frequency of 60 kHz is used by several nations, such as the United States, England, and Japan, for extremely accurate time and precision frequency signals. (77.5 kHz is used inGermany.) Many commercial appliances (such as self-setting clocks) sold since approximately 2000 have a VLF receiver capable of receiving these signals, which penetrate indoors more effectively than mediumwave or shortwave signals.
Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 meters. The United States, Russian, British, Swedish, and Indian navies communicate withsubmarines on these frequencies. In addition, Royal Navy nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor the BBC Radio 4 transmission on 198 kHz in waters near the U.K. It is rumoured that they are to construe a sudden halt in transmission as an indicator that the U.K. is under attack, whereafter their sealed orders go into place.
Longwave transmitting antennas take up large amounts of space, and have been the cause of controversy in the United States and Europe due to fears over proximity to high-power radio waves.