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Very low frequency or VLF is the ITU designation〔(【引用サイトリンク】url=http://www.itu.int/dms_pubrec/itu-r/rec/v/R-REC-V.431-7-200005-I!!PDF-E.pdf )〕 for radio frequencies (RF) in the range of 3 kHz to 30 kHz and wavelengths from 10 to 100 kilometres. Due to its limited bandwidth, audio (voice) transmission is highly impractical, 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 penetrate about 40 meters into saltwater, they are used for military communication with submarines. 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). ==Propagation characteristics== 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 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. VLF waves have very low path attenuation, 2-3 dB per 1000 km,〔 with little of the "fading" experienced at higher frequencies,〔 This is because VLF waves are reflected from the bottom of the ionosphere, while higher frequency shortwave signals are returned to Earth from higher layers in the ionosphere, the F1 and F2 layers, by a refraction process, and spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are very stable and reliable, and are used for long distance communication. Propagation distances of 5000 to 20000 km have been realized.〔 However, atmospheric noise (sferics) is high in the band,〔 including such phenomena as "whistlers", caused by lightning. VLF waves can penetrate seawater to a depth of at least 10 to 40 meters (30 to 130 feet), depending on the frequency employed and the salinity of the water, so they are used to communicate with submarines. VLF waves at certain frequencies have been found to cause electron precipitation (TIPER). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「very low frequency」の詳細全文を読む スポンサード リンク
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