Extremely Low Frequency
Electromagnetic waves in the ELF frequency range (3–300 Hz) (see also SLF) can penetrate seawater to depths of hundreds of meters, allowing communication with submarines at their operating depths. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The US Navy's system (called Seafarer) operated at 76 hertz, the Soviet/Russian system (called ZEVS) at 82 hertz. The latter corresponds to a wavelength of 3,658.5 kilometers. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be constructed, as it would spread across a large country.
Instead, one has to find an area with very low ground conductivity (a requirement opposite to usual radio transmitter sites), bury two huge electrodes in the ground at different sites, and then feed lines to them from a station in the middle, in the form of wires on poles. Although other separations are possible, 60 kilometers is the distance used by the ZEVS transmitter located near Murmansk. As the ground conductivity is poor, the current between the electrodes will penetrate deep into the Earth, essentially using a large part of the globe as an antenna. The antenna length in Republic, Michigan was approximately 52 kilometers (32 mi). The antenna is very inefficient. To drive it, a dedicated power plant seems to be required, although the power emitted as radiation is only a few watts. Its transmission can be received virtually anywhere. A station in Antarctica at 78°S 167°W was noticed when the Soviet Navy put their ZEVS antenna into operation.
Due to the technical difficulty of building an ELF transmitter, only the US and the Russian Navy owned such systems. Until it was dismantled in late September 2004, the American Seafarer, later called Project ELF system (76 Hz) consisted of two antennas, located at Clam Lake, Wisconsin (since 1977) and at Republic, Michigan in the Upper Peninsula (since 1980). The Russian antenna (ZEVS, 82 Hz) is installed at the Kola Peninsula near Murmansk. It was noticed in the West in the early 1990s. The British Royal Navy once considered building their own transmitter at Glengarry Forest, Scotland, but the project was canceled.
Read more about this topic: Communication With Submarines
Famous quotes containing the words extremely low, extremely and/or frequency:
“The Heavens. Once an object of superstition, awe and fear. Now a vast region for growing knowledge. The distance of Venus, the atmosphere of Mars, the size of Jupiter, and the speed of Mercury. All this and more we know. But their greatest mystery the heavens have kept a secret. What sort of life, if any, inhabits these other planets? Human life, like ours? Or life extremely lower in the scale. Or dangerously higher.”
—Richard Blake, and William Cameron Menzies. Narrator, Invaders from Mars, at the opening of the movie (1953)
“My plan of instruction is extremely simple and limited. They learn, on week-days, such coarse works as may fit them for servants. I allow of no writing for the poor. My object is not to make fanatics, but to train up the lower classes in habits of industry and piety.”
—Hannah More (1745–1833)
“One is apt to be discouraged by the frequency with which Mr. Hardy has persuaded himself that a macabre subject is a poem in itself; that, if there be enough of death and the tomb in one’s theme, it needs no translation into art, the bold statement of it being sufficient.”
—Rebecca West (1892–1983)