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ground dipole
In radio communication, a ground dipole,〔, p.1692, on ( VLF Group website, Stanford Univ. )〕 also referred to as an earth dipole antenna, transmission line antenna,〔 and in technical literature as a horizontal electric dipole (HED),〔〔 on ( Federation of American Scientists website )〕 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 frequency range of 3 Hz to 3 kHz, commonly called ELF waves〔 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.〔〔(【引用サイトリンク】title=Navy gets new facility to communicate with nuclear submarines prowling underwater )〕 The U.S. facilities were used between 1985 and 2004 but are now decommissioned.〔
==Antennas at ELF frequencies==
Although the official ITU definition of extremely low frequencies is 3 Hz to 30 Hz, the wider band of frequencies of 3 Hz to 3 kHz is used for ELF communication and are commonly called ELF waves.〔Liemohn, Michael W. and A. A. Chan, "(Unraveling the Causes of Radiation Belt Enhancements )". EOS, Transactions, American Geophysical Union, Volume 88, Number 42, 16 October 2007, pages 427-440. Republished by NASA and accessed online, 18 November 2015.〕), 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,〔λ = c/f = 3×108 m/s / 80 Hz = 3750 km〕 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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