English: Map showing the locations of the two extremely low frequency (ELF) transmitters in Clam Lake, Wisconsin and Republic, Michigan, USA, used by the U.S. Navy from 1989 to 2004 to communicate with submarines. From a U.S. Navy public relations pamphlet. The red lines show the paths of the huge ground dipole antennas, consisting of 14 to 28 mile power transmission lines grounded at the ends, which radiated the ELF waves. This is one of only two ELF transmitter facilities that have been constructed; the other is the Russian Navy ZEVS transmitter on the Kola Penninsula in northern Russia.
The radiation pattern of each ground dipole antenna is maximum along the axis of the transmission line, so each transmitter had perpendicular ground dipoles, to allow it to transmit in any direction. The Clam Lake, WI transmitter (left) consisted of two crossed 14 mi. long ground dipoles, one oriented north-south, the other east-west. The Republic, MI transmitter consisted of two 14 mi. transmission lines oriented east-west, and one 28 mi. line oriented north-south (the specific shape of the antenna has no significance and was dictated by land availability). Each site had a 1 megawatt transmitter which drove the lines with currents of 150-300 amperes. The two sites were normally operated phase-synchronized as one antenna, with an output power of 8 watts, but could operate separately to improve survivability.