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 a ground wave 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 ions in the 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, so they 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 roughly 10 to 40 metres (30 to 130 feet), depending on the frequency employed and the salinity of the water, so they are used to communicate with submarines when they are at shallow depths.
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