Extremely Low Frequencies
7 hours ago
- #military history
- #submarine communication
- #VLF and ELF technology
- Submarines were first used in combat during the US Civil War, but were primitive and limited.
- World War I saw significant advances in submarine technology, especially with German U-boats, establishing submarine warfare.
- Early submarines had limited submerged endurance due to diesel-electric or gasoline power, requiring surfacing for communication.
- Radio communication underwater was challenging because seawater blocks high-frequency radio waves, forcing submarines to surface.
- The Navy experimented with radio on submarines like the C-class and D-class in 1909, but initial tests were problematic.
- Engineers developed floating antenna buoys around 1915 to allow submarines to communicate while submerged, though detection was a risk.
- John Willoughby and Percival Lowell discovered that very low frequency (VLF) radio waves could penetrate seawater, leading to long-wave radio for submarines.
- The Navy adopted VLF radio for submarine communications in the early 1920s, with stations like NSS in Annapolis becoming key for command and control.
- VLF stations, such as Cutler in Maine, were built with massive antennas and high-power transmitters to communicate with submerged submarines.
- VLF signals (3-30 kHz) penetrate seawater better than higher frequencies, allowing reception up to about 100 feet below the surface.
- VLF antennas are large due to long wavelengths, and magnetic loop antennas are efficient for reception but not for transmission.
- VLF has limited bandwidth, restricting it to low-speed communications like Morse code or simple digital signals.
- The development of nuclear-powered submarines in the 1950s allowed indefinite submersion, increasing the need for stealthy communication.
- Extremely Low Frequency (ELF) radio (3-30 Hz) was explored for deeper underwater communication, with wavelengths up to 2,300 miles.
- ELF antennas, like ground dipoles, are massive and inefficient, requiring huge power for minimal radiated output.
- Project Sanguine (1968) proposed a hardened ELF network with over 100 transmitters covering thousands of square miles, but was canceled due to cost, public opposition, and technical challenges.
- Project ELF, a scaled-down version, became operational in 1989 with sites in Wisconsin (Clam Lake) and Michigan (Republic), transmitting at about 8 watts.
- ELF acted as a 'pager' system, alerting submarines to surface for VLF messages, with very slow data rates (e.g., one letter per five minutes).
- The US Navy shut down ELF in 2004, citing improved technologies and changing requirements, while Russia, China, and India maintain ELF capabilities.
- ELF faced political and environmental opposition, including health concerns and protests, and inspired popular culture references like an episode of The X-Files.