English: Diagram of ground dipole antenna used to transmit extremely low frequency (ELF) radio waves, showing how it works. This is modeled on the U.S. Navy ELF radio transmitter at Clam Lake, Wisconsin, which was used to communicate with submerged submarines at a frequency of 76 Hz from 1989 to 2004. The antenna consists of two electrodes buried in the Earth (G), 14 miles (23.5 km) apart, linked by overhead transmission lines resembling ordinary power distribution lines to a power plant transmitter (P). The system functions as a giant vertical loop antenna in which the earth serves as part of the loop. The power plant drives the antenna with 300 amperes alternating current at 76 Hz which flows through one transmission line, in looping currents (I) deep in bedrock from one ground electrode to the other, and back through the other transmission line (the drawing shows only one direction of the alternating current). The AC current creates an alternating magnetic field (H) which radiates ELF waves (yellow). The radiation pattern is directional, with two lobes (maxima) off the ends of the wire. This drawing was made with information from Jones, David Llanwyn (1985-07-04). "Sending signals to submarines". New Scientist 26 (1463): 37-41. London: Holborn Publishing Group. Retrieved on February 17, 2012., among other sources.