Ground dipole

[4][5] It is the only type of transmitting antenna that can radiate practical amounts of power in the frequency range of 3 Hz to 3 kHz, commonly called ELF waves.

Besides small research and experimental antennas,[5][6] four full-scale ground dipole installations are known to have been constructed; two by the U.S. Navy at Republic, Michigan and Clam Lake, Wisconsin,[2][7][8] one by the Russian Navy on the Kola peninsula near Murmansk, Russia,[8][9][10] and one in India at the INS Kattabomman naval base.

[13] The frequency used in the U.S. and Russian transmitters, about 80 Hz,[1][14] generates waves 3750 km (2300 miles) long,[a][15] roughly one quarter of the Earth's diameter.

Ordinary types of antenna (half-wave dipoles and quarter-wave monopoles) cannot be built for such extremely long waves because of their size.

Fortunately, the attenuation of ELF waves with distance is so low (1–2 dB per 1000 km)[5] that a few watts of radiated power is enough to communicate worldwide.

Submarines when submerged are shielded by seawater from all ordinary radio signals, and therefore are cut off from communication with military command authorities.

Cristofilos proposed applying currents to the Earth to create a vertical loop antenna, and it became clear that this was the most practical design.

It consists of two widely separated electrodes (G) buried in the ground, connected by overhead transmission cables to a transmitter (P) located between them.

Due to their low frequency, ELF waves have a large skin depth and can penetrate a significant distance through earth, so it doesn't matter that half the antenna is under the ground.

[5] Christofilos found that the lower the electrical conductivity of the underlying rock, the deeper the current will go, and the larger the effective loop area.

After initially considering several larger systems (Project Sanguine), the U.S. Navy constructed two ELF transmitter facilities, one at Clam Lake, Wisconsin and the other at Republic, Michigan, 145 miles apart, transmitting at 76 Hz.

[2] However, due to the low attenuation of ELF waves this tiny radiated power was able to communicate with submarines over about half the Earth's surface.

[8] The Russian Navy operates an ELF transmitter facility, named ZEVS ("Zeus"), to communicate with its submarines, located 30 km southeast of Murmansk on the Kola peninsula in northern Russia.

[9][14] It reportedly consists of two parallel ground dipole antennas 60 km long, driven at currents of 200–300 amperes.

The U.S. Navy Clam Lake, Wisconsin ELF transmitter in 1982. Sections of the power lines that make up the two crossed ground dipole antennas can be seen passing through the forest at lower left.
Ground dipole antenna, similar to the U.S. Clam Lake antennas, showing how it works. The alternating current , I , is shown flowing in one direction only through the loop for clarity.
Map showing location of the US Navy ELF transmitters. The red lines show the paths of the ground dipole antennas. The Clam Lake facility (left) had two crossed 14 mile (23 km) ground dipoles. The Republic facility had two 14 mile dipoles oriented east–west, and one 28 mile dipole oriented north–south. The different shapes of the dipoles was dictated by land availability, and did not indicate a difference in design.