VLF

Propagation characteristics

VLF waves have very low path attenuation, 2–3 dB per 1,000 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 5,000–20,000 km have been realized. However, atmospheric noise ("sferics") is high in the band, including such phenomena as "whistlers", caused by lightning.