Extremely Low Frequencies (ELF)

ELF and Band Designators

In some references, the entire frequency range between 3 Hz and 3kHz is called ELF, with ULF applying to all frequencies below 3 Hz.

The Navy's ELF Communication System

The degree to which a signal is attenuated depends on its frequency, however. The lower the frequency, the more deeply a signal can be received in sea water. In order to receive conventional radio transmissions a submarine must travel at slow speeds and be near the surface of the water. Both of these situations make a submarine more susceptible to enemy detection. Frequencies in the ELF range, however, can be received considerably deeper, and broadcasts using this mode provide a primary link between the nation's commander-in-chief and the submarine force.

One of the great difficulties associated with the use of ELF for communication purposes, is the problem of generating a useful signal. The physical size of an antenna that can produce a useable signal with reasonable efficiency is inversely proportional to the frequency. For example, an antenna useful for cellular telephone frequencies, need only be several inches long to be completely effective. At ELF, on the other hand, a reasonably efficient antenna must be quite large.

The ELF system, which became operational in 1989, uses two transmitting antennas, one in Wisconsin and one in Michigan. The two sites must operate simultaneously to meet worldwide coverage requirements. Each antenna looks like a power line, mounted on wooden poles. The Wisconsin antenna consists of two lines, each about 14 miles long. The Michigan antenna uses three lines, two about 14 miles long and one about 28 miles long. Each site has a transmitter building near the antenna. The transmitter facility in Michigan uses about six acres of land and the one in Wisconsin about two acres. The operating frequency is 76 Hz.

The construction required no relocation of people or buildings. The antenna location in State and National forests avoided buildings, historic sites, villages, and towns. Construction contractors coordinated extensively with the Michigan Department of Natural Resources and the U.S. Forest Service to avoid rare vegetation and to repopulate the easement with local flora.

The National Academy of Sciences reviewed the ELF program in 1977 for possible ecological effects. While it found none at that time, the study did recommend that the Navy conduct an ecological monitoring program. As a result, in the last 10 years, several universities, funded by the Navy, conducted independent studies to look for ecological effects of ELF. The studies found no adverse effects on animals, plants, or micro-organisms at the ELF system test sites. Much of this research has already been published in scientific journals. The National Academy of Sciences (NAS) is reviewing the results for proper data analysis and scientific procedure. The NAS report is due in October of 1995. The National Technical Information Service will provide any report generated by the ELF program upon request.

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