VLF

Very Low Frequency (VLF) refers to the radio frequency band from 3 kHz to 30 kHz. At these frequencies, wavelengths range from about 100 kilometers down to 10 kilometers. VLF transmissions require large antennas and high-power transmitters, and reception typically uses sensitive, low-frequency receivers. The band supports very long-range propagation, but offers limited data capacity.

VLF signals propagate primarily through the earth–ionosphere waveguide, combining ground-wave propagation along the surface with multiple reflections between the Earth and the ionosphere. This mode enables reception over intercontinental distances with relatively low attenuation, though propagation is strongly influenced by ground conductivity and ionospheric conditions. The result is reliable, long-range communication in certain circumstances, but highly channel-limited for modern data services.

Applications of VLF include military and government communications, especially for communicating with submerged submarines, where higher-frequency signals would be rapidly attenuated by seawater. VLF is also used in geophysics and atmospheric science, where VLF electromagnetic methods (VLF-EM) help map subsurface conductivity, and where natural VLF phenomena such as lightning sferics are studied to probe the ionosphere and the near-Earth environment. Timekeeping and navigation services have historically leveraged the band as part of legacy networks and reference transmissions.

Limitations of the VLF band include very low data rates, the need for specialized infrastructure and licensing, and sensitivity to natural and anthropogenic noise. Despite these constraints, VLF remains valuable for specific long-range and sensing applications where its propagation properties are advantageous.