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Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 to 30 kHz. Since there is
not much bandwidth in this band of the radio spectrum, only the very simplest
signals are used, such as for radionavigation. Because VLF can
penetrate water to a depth of 20 meters (66
feet), they are used to communicate with submarines near the surface. (ELF is
used for fully submerged vessels.)
Many natural radio emissions, such as
whistlers, can be also heard in this band.
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