Physics Questions People Ask Fermilab
Extremely Low Frequency system
In Michigan, there is an ELF (extremely low frequency) underground cable. My students asked what it does. Can you help?
Dear Ms. Mahon:
Thank you for your letter inquiring about the Extremely Low Frequency system. Fermilab is not involved in this project, but I think I still can help you.
Searching the Internet, I found information on the ELF system operated by the U.S. Navy. It is actually not an underground cable but rather an above-ground antenna consisting of two "branches." Electrical currents flow from one "branch" through the ground to the other "branch." Flowing back and forth, the currents create oscillating fields.
Because of the current flowing through the ground, you thought that there is a cable. Instead, the Navy just relies on the ground. Choosing sites with low conductivity rock near the surface, the currents penetrate the ground hundreds of meters before returning to the opposite branch above ground. Since the total length of the ELF antenna (including the long path through the ground) is so huge, the antenna is able to create electromagnetic waves with an extremely low frequency of 76 Hertz. (Radio waves are usually measured in thousands of Hertz (kHz, AM stations) or millions of Hertz (MHz, FM stations).)
The Navy uses the ELF waves to contact submarines deep under water, depths that normal radio waves cannot penetrate. (Water absorbs high frequencies. You may have noticed this when diving.) The low frequency, however, makes for a terrible radio signal that cannot broadcast much useful information. (Radio signals get modulated either in amplitude (AM) or frequency (FM) to broadcast signals.) So the Navy only uses the ELF system to signal their submarines to come closer to the surface, and then broadcasts the real message by other means. With the ELF system, the Navy is basically able to "knock on the submarines' doors."
For additional information, please check out the links below. Again, Fermilab is not involved in the ELF system or any other military research.
Fermilab, Public Affairs
The ELF system operated by the U.S. Navy with two transmitting antennas, one in Wisconsin and one in Michigan:
Details on the ELF antenna in Wisconsin http://enterprise.spawar.navy.mil/spawarpublicsite/docs/fs_clam_lake_elf.pdf
Effectiveness of ELF system is disputed: http://www.greenscissors.org/international-military/elf.htm
- Last modified
- email Fermilab