In radio communication, a ground dipole, also referred to as an earth dipole antenna, transmission line antenna, and in technical literature as a horizontal electric dipole (HED), is a huge, specialized type of radio antenna that radiates extremely low frequency (ELF) electromagnetic waves. It is the only type of transmitting antenna that can radiate practical amounts of power in the ELF frequency range of 3 Hz to 3 kHz. A ground dipole consists of two ground electrodes buried in the earth, separated by tens to hundreds of kilometers, linked by overhead transmission lines to a power plant transmitter located between them. Alternating current electricity flows in a giant loop between the electrodes through the ground, radiating ELF waves, so the ground is part of the antenna. To be most effective, ground dipoles must be located over certain types of underground rock formations. The idea was proposed by U.S. Dept. of Defense physicist Nicholas Christofilos in 1959.
Although small ground dipoles have been used for years as sensors in geological and geophysical research, their only use as antennas has been in a few military ELF transmitter facilities to communicate with submerged submarines. Besides small research and experimental antennas, 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, one by the Russian Navy on the Kola peninsula near Murmansk, Russia. and one in India at the INS Kattabomman naval base. The U.S. facilities were used between 1985 and 2004 but are now decommissioned.
Antennas at ELF frequencies
ELF frequencies (according to one of several definitions) range from 3 Hz to 3 kHz, with corresponding wavelengths from 100,000 km to 100 km. The frequency used in the U.S. and Russian transmitters, about 80 Hz, generates waves 3750 km (2300 miles) long, roughly one quarter of the Earth's diameter. ELF waves have been used in very few manmade communications systems because of the difficulty of building efficient antennas for such long waves. Ordinary types of antenna (half-wave dipoles and quarter-wave monopoles) cannot be built for such extremely long waves because of their size. A half wave dipole for 80 Hz would be 1162 miles long. So even the largest practical antennas for ELF frequencies are very electrically short, very much smaller than the wavelength of the waves they radiate. The disadvantage of this is that the efficiency of an antenna drops as its size is reduced below a wavelength. An antenna's radiation resistance, and the amount of power it radiates, is proportional to (L/λ)2 where L is its length and λ is the wavelength. So even physically large ELF antennas have very small radiation resistance, and so radiate only a tiny fraction of the input power as ELF waves; most of the power applied to them is dissipated as heat in various ohmic resistances in the antenna. ELF antennas must be tens to hundreds of kilometers long, and must be driven by powerful transmitters in the megawatt range, to produce even a few watts of ELF radiation. Fortunately, the attenuation of ELF waves with distance is so low (1 - 2 dB per 1000 km) that a few watts of radiated power is enough to communicate worldwide.
A second problem stems from the required polarization of the waves. ELF waves only propagate long distances in vertical polarization, with the direction of the magnetic field lines horizontal and the electric field lines vertical. Vertically oriented antennas are required to generate vertically polarized waves. Even if sufficiently large conventional antennas could be built on the surface of the Earth, these would generate horizontally polarized, not vertically polarized waves.
In 1958, the realization that ELF waves could penetrate seawater led U.S. physicist Nicholas Christofilos to suggest that the U.S. Navy use them to communicate with submarines. The U.S. military researched many different types of antenna for use at ELF frequencies. Cristofilos proposed applying currents to the Earth to create a vertical loop antenna, and it became clear that this was the most practical design. The feasibility of the ground dipole idea was tested in 1962 with a 42 km leased power line in Wyoming, and in 1963 with a 176 km prototype wire antenna extending from West Virginia to North Carolina.
How a ground dipole works
A ground dipole functions as an enormous vertically oriented loop antenna (see drawing, right). It consists of two widely separated electrodes (G) buried in the ground, connected by overhead transmission cables to a transmitter (P) located between them. The alternating current from the transmitter (I) travels in a loop through one transmission line, kilometers deep into bedrock from one ground electrode to the other, and back through the other transmission line. This creates an alternating magnetic field (H) through the loop, which radiates ELF waves. 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. The axis of the magnetic field produced is horizontal, so it generates vertically polarized waves. The radiation pattern of the antenna is directional, with two lobes (maxima) in the plane of the loop, off the ends of the transmission lines. In the U.S. installations two ground dipoles are used, oriented perpendicular to each other, to allow transmission in all directions.
The amount of power radiated by a loop antenna is proportional to (IA)2, where I is the AC current in the loop and A is the area enclosed, To radiate practical power at ELF frequencies, the loop has to carry a current of hundreds of amperes and enclose an area of at least several square miles. 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. Radio frequency current will penetrate into the ground to a depth equal to the skin depth of the ground at that frequency, which is inversely proportional to the square root of ground conductivity σ. The ground dipole forms a loop with effective area of A = Lδ / √2, where L is the total length of the transmission lines and δ is the skin depth. So ground dipoles are sited over low conductivity underground rock formations (this contrasts with ordinary radio antennas, which require good earth conductivity for a low resistance ground connection for their transmitters). The two U.S. Navy antennas were located in the Upper Peninsula of Michigan, on the Canadian Shield (Laurentian Shield) formation, which has unusually low conductivity of 2×10−4 siemens/meter. resulting in an increase in antenna efficiency of 20 dB. The earth conductivity at the site of the Russian transmitter is even lower.
Because of their lack of civilian applications, little information about ground dipoles is available in antenna technical literature.
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. They could operate independently, or phase synchronized as one antenna for greater output power. The Clam Lake site, the initial test facility, transmitted its first signal in 1982 and began operation in 1985, while the Republic site became operational in 1989. The total radiated output power of both sites working together was 8 watts. However due of the low attenuation of ELF waves this tiny radiated power was able to communicate with submarines over about half the Earth's surface.
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. Signals from it were detected in the 1990s at Stanford University and elsewhere. It normally operates at 82 Hz, using MSK (minimum shift keying) modulation. although it reportedly can cover the frequency range from 20 to 250 Hz. It reportedly consists of two parallel ground dipole antennas 60 km long, driven at currents of 200 to 300 amperes. Calculations from intercepted signals indicate it is 10 dB more powerful than the U.S. transmitters. Unlike them it is used for geophysical research in addition to military communications.
The total power radiated by a ground dipole is
- f is the frequency
- I is the RMS current in the loop
- L is the length of the transmission line
- c is the speed of light
- h is the height above ground of the ionosphere D layer
- σ is the ground conductivity
The radiated power of an electrically small loop antenna normally scales with the fourth power of the frequency, but at ELF frequencies the effects of the ionosphere result in a less severe reduction in power proportional to the square of frequency.
Ground dipoles are not needed for reception of ELF signals. The requirements for receiving antennas at ELF frequencies are less stringent than transmitting antennas, because the signal to noise ratio in ELF receivers is dominated by the large atmospheric noise in the band. The level of signal strength at the antenna is far above the noise in the receiver circuit, so small inefficient receiving antennas can be used. Various types of coil and ferrite loop antennas have been used for reception.
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