VLF

Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3 kHz to 30 kHz and corresponding wavelengths from 100 to 10 kilometres, respectively. The band is also known as the myriametre band or myriametre wave as the wavelengths range from one to ten myriametres (an obsolete metric unit equal to 10 kilometres). Due to its limited bandwidth, audio (voice) transmission is highly impractical in this band, and therefore only low data rate coded signals are used. The VLF band is used for a few radio navigation services, government time radio stations (broadcasting time signals to set radio clocks) and for secure military communication. Since VLF waves can penetrate at least 40 meters (120 ft) into saltwater, they are used for military communication with submarines.

Because of their large wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and can propagate as ground waves following the curvature of the Earth. The main mode of long distance propagation is an Earth-ionosphere waveguide mechanism. The Earth is surrounded by a conductive layer of electrons and ions in the upper atmosphere, the ionosphere D layer at 60 km altitude, which reflects VLF radio waves. The conductive ionosphere and the conductive Earth, form a horizontal "duct" a few VLF wavelengths high, which acts as a waveguide confining the waves so they don't escape into space. The waves travel in a zigzag path around the Earth, reflected alternately by the Earth and the ionosphere, in TM (transverse magnetic) mode.