File:Clam Lake ELF.jpg

English: An aerial view of the U.S. Navy's Clam Lake Extremely Low Frequency (ELF) radio transmitter facility. This transmitted ELF radio signals at a frequency of 76 Hz to communicate with submerged nuclear submarines around the world. The transmitter powered two perpendicular "ground dipole" antennas each consisting of a 14 mile transmission line grounded at each end. Sections of the transmission lines can be seen passing through the forest at lower left. The transmission lines were driven with a current of about 300 amps, which flowed deep in the ground between the ends of the lines, in a huge loop. The input power to the antennas was about 1 megawatt, and they radiated about 2 watts of ELF radiation. The Clam Lake facility became operational in 1985 and was shut down in 2004. This photo was taken in 1982 before it became operational, when it was known as the Wisconsin Test Facility.