Applications
The frequency range below 9 kHz is not allocated by the International Telecommunication Union and may be used in some nations license-free.
VLF is used to communicate with submarines near the surface (for example using the transmitter DHO38), while ELF is used for deeply-submerged vessels. VLF is also used for radio navigation beacons (alpha) and time signals (beta). VLF is also used in electromagnetic geophysical surveys.
In the USA, the time signal station WWVL began transmitting a 500 W signal on 20 kHz in August 1963. It used Frequency Shift Keying (FSK) to send data, shifting between 20 kHz and 26 kHz. The WWVL service was discontinued in July 1972.
The very long wave transmitter SAQ at Grimeton near Varberg in Sweden can be visited by the public at certain times, such as on Alexanderson Day.
Historically, this band was used for transcontinental radio communication during the wireless telegraphy era between about 1900 and 1925. Nations built networks of high power LF and VLF radio stations which transmitted text information by Morse code, to communicate with their colonies and naval fleets. Early attempts were made to use radiotelephone using amplitude modulation and single-sideband modulation within the band starting from 20 kHz, but the result was unsatisfactory because the available bandwidth was insufficient to contain the sidebands.
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