extremely low frequency
The band of radio frequencies from 3 to 30 hertz.
ELF: radio waves so slow they measure in seconds per cycle
Extremely low frequency, or ELF, denotes electromagnetic radiation in the 3 to 30 hertz band. At these frequencies, one complete wave cycle takes between 33 milliseconds and one third of a second. This sits far below the radio spectrum most people know; AM broadcast radio operates around 1 megahertz (one million hertz), making ELF roughly a billion times slower.
ELF propagation differs radically from higher frequencies. The long wavelengths, ranging from 10,000 to 100,000 kilometers, pass through seawater with minimal attenuation, penetrate rock and soil effectively, and refract around the Earth's curvature rather than reflecting off the ionosphere. This makes ELF uniquely suited for communicating with submerged submarines, which cannot receive standard radio signals underwater.
Military navies have operated ELF transmitters for this reason. The US Navy operated a facility in Wisconsin transmitting on 76 hertz; similar installations existed in other countries. Transmitter antennas for ELF are enormous, often several kilometers long, because effective radiation requires antenna dimensions comparable to the wavelength. The power requirements are also substantial, as low frequency radiation is inherently inefficient.
Biological and environmental questions
ELF entered public consciousness partly through debate over health effects. Power lines and electrical equipment emit ELF radiation as a byproduct, typically at 50 or 60 hertz (the standard AC supply frequencies in different countries). Scientific literature has examined whether chronic exposure to ELF fields poses health risks, though no consensus mechanism has been established and regulatory standards vary widely.
In industrial contexts, ELF measurement matters in electromagnetic compatibility work, power quality assessment, and occupational exposure monitoring near high-voltage transmission lines. Technicians use specialized instruments to characterize ELF fields, distinguishing them from the higher frequency components that may be present in electrical noise on the same plant or grid.