Ground dipole

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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. Clam Lake ELF.jpg
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.

In radio communication, a ground dipole, [1] also referred to as an earth dipole antenna, transmission line antenna, [1] and in technical literature as a horizontal electric dipole (HED), [1] [2] [3] is a huge, specialized type of radio antenna that radiates extremely low frequency (ELF) electromagnetic waves. [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. [5] 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. [1] [5] 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. [5] The idea was proposed by U.S. Dept. of Defense physicist Nicholas Christofilos in 1959. [5]

Contents

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, [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. [11] [12] The U.S. facilities were used between 1985 and 2004 but are now decommissioned. [8]

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 with corresponding wavelengths from 100,000 km to 100 km [1] is used for ELF communication and are commonly called ELF waves. [13] The frequency used in the U.S. and Russian transmitters, about 80 Hz, [1] [14] generates waves 3750 km (2300 miles) long, [lower-alpha 1] [15] 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. [1] The disadvantage of this is that the efficiency of an antenna drops as its size is reduced below a wavelength. [1] 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. [5] 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) [5] that a few watts of radiated power is enough to communicate worldwide. [2]

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. [1] 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.

History

Submarines when submerged are shielded by seawater from all ordinary radio signals, and therefore are cut off from communication with military command authorities. VLF radio waves can penetrate 50–75 feet into seawater and have been used since WWII to communicate with submarines, but the submarine must rise close to the surface, making it vulnerable to detection. In 1958, the realization that ELF waves could penetrate deeper into seawater, to normal submarine operating depths led U.S. physicist Nicholas Christofilos to suggest that the U.S. Navy use them to communicate with submarines. [7] [15] 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. [1] [15] 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. [5] [15]

How a ground dipole works

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. Ground dipole ELF antenna.svg
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.

A ground dipole functions as an enormous vertically oriented loop antenna [5] [16] (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, a dipole pattern, with two lobes (maxima) in the plane of the loop, off the ends of the transmission lines. [3] [5] In the U.S. installations two ground dipoles are used, oriented perpendicular to each other, to allow the beam to be steered in any direction by altering the relative phase of the currents in the antennas.

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, [5] 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. [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. [2] [5] 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 = 1/√2 L δ, where L is the total length of the transmission lines and δ is the skin depth. [5] [14] Thus, 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, [2] [17] which has unusually low conductivity of 2×10−4 siemens/meter [5] resulting in an increase in antenna efficiency of 20 dB. [3] The earth conductivity at the site of the Russian transmitter is even lower. [14]

Because of their lack of civilian applications, little information about ground dipoles is available in antenna technical literature.

U.S. Navy ELF antennas

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. US Navy ELF transmitter map.png
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.

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] [4] They could operate independently, or phase synchronized as one antenna for greater output power. [4] The Clam Lake site, the initial test facility, transmitted its first signal in 1982 [4] and began operation in 1985, while the Republic site became operational in 1989. With an input power of 2.6 megawatts, the total radiated ELF output power of both sites working together was 8 watts. [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. [18]

Both transmitters were shut down in 2004. [8] [19] The official Navy explanation was that advances in VLF communication systems had made them unnecessary. [8]

Russian Navy ZEVS antennas

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] [10] Signals from it were detected in the 1990s at Stanford University and elsewhere. [10] [14] It normally operates at 82 Hz, using MSK (minimum shift keying) modulation. [10] although it reportedly can cover the frequency range from 20 to 250 Hz. [9] [14] It reportedly consists of two parallel ground dipole antennas 60 km long, driven at currents of 200–300  amperes. [10] [14] Calculations from intercepted signals indicate it is 10 dB more powerful than the U.S. transmitters. [14] Unlike them it is used for geophysical research in addition to military communications. [9] [10]

Indian Navy antennas

The Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base, in Tamil Nadu, to communicate with its Arihant class and Akula class submarines. [11] [12]

Radiated power

The total power radiated by a ground dipole is [5]

where 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’s D layer, and 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.

Receiving antennas

Ground dipoles are not needed for reception of ELF signals, although some radio amateurs use small ones for this purpose. Instead, various loop and ferrite coil antennas have been used for reception.

The requirements for receiving antennas at ELF frequencies are far less stringent than transmitting antennas: [lower-alpha 2] In ELF receivers, noise in the signal is dominated by the large atmospheric noise in the band. Even the tiny signal captured by a small, inefficient receiving antenna contains noise that greatly exceeds the small amount of noise generated in the receiver itself. [lower-alpha 3] Because the outside noise is what limits reception, very little power from the antenna is needed for the intercepted signal to overwhelm the internal noise, and hence small receive antennas can be used with no disadvantage.

See also

Footnotes

  1. ↑ λ = c/f = 3×108 m/s/80 Hz = 3750 km
  2. ↑ The signal-to-noise ratio (SNR) is the limiting factor in all radio reception, and the limiting noise comes both from outside the receiver and from inside the receiver's own circuitry. The constraint this places on receiving antennas is they must intercept a strong enough signal to stand out from the external and internal background noise.
  3. ↑ Atmospheric noise is predominant at all frequencies below about 1,500 kHz.

Related Research Articles

Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth beyond the visible horizon. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in radio propagation are not confined to the surface.

<span class="mw-page-title-main">Radio wave</span> Type of electromagnetic radiation

Radio waves are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths greater than 1 millimeter, about the diameter of a grain of rice. Like all electromagnetic waves, radio waves in a vacuum travel at the speed of light, and in the Earth's atmosphere at a slightly slower speed. Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects, and are part of the blackbody radiation emitted by all warm objects.

<span class="mw-page-title-main">Medium wave</span> Radio transmission using wavelengths 200-1000 m

Medium wave (MW) is a part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the daytime, reception is usually limited to more local stations, though this is dependent on the signal conditions and quality of radio receiver used. Improved signal propagation at night allows the reception of much longer distance signals. This can cause increased interference because on most channels multiple transmitters operate simultaneously worldwide. In addition, amplitude modulation (AM) is often more prone to interference by various electronic devices, especially power supplies and computers. Strong transmitters cover larger areas than on the FM broadcast band but require more energy and longer antennas. Digital modes are possible but have not reached momentum yet.

<span class="mw-page-title-main">Very low frequency</span> The range 3–30 kHz of the electromagnetic spectrum

Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten myriameters. Due to its limited bandwidth, audio (voice) transmission is highly impractical in this band, and therefore only low data rate coded signals are used. The VLF band is used for a few radio navigation services, government time radio stations and for secure military communication. Since VLF waves can penetrate at least 40 meters (131 ft) into saltwater, they are used for military communication with submarines.

Low frequency (LF) is the ITU designation for radio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as the kilometre band or kilometre waves.

<span class="mw-page-title-main">Medium frequency</span> The range 300-3000 kHz of the electromagnetic spectrum

Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300 kilohertz (kHz) to 3 megahertz (MHz). Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one hectometers. Frequencies immediately below MF are denoted as low frequency (LF), while the first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, and transoceanic air traffic control.

<span class="mw-page-title-main">High frequency</span> The range 3-30 MHz of the electromagnetic spectrum

High frequency (HF) is the ITU designation for the band of radio waves with frequency between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters. Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (3.95–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.

<span class="mw-page-title-main">Antenna (radio)</span> Electrical device

In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.

Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for amateur radio communications, international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems.

<span class="mw-page-title-main">Extremely low frequency</span> The range 3-30 Hz of the electromagnetic spectrum

Extremely low frequency (ELF) is the ITU designation for electromagnetic radiation with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively. In atmospheric science, an alternative definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower-frequency electromagnetic oscillations are considered to lie in the ULF range, which is thus also defined differently from the ITU radio bands.

Super low frequency (SLF) is the ITU designation for electromagnetic waves in the frequency range between 30 hertz and 300 hertz. They have corresponding wavelengths of 10,000 to 1,000 kilometers. This frequency range includes the frequencies of AC power grids. Another conflicting designation which includes this frequency range is Extremely Low Frequency (ELF), which in some contexts refers to all frequencies up to 300 hertz.

Communication with submarines is a field within military communications that presents technical challenges and requires specialized technology. Because radio waves do not travel well through good electrical conductors like salt water, submerged submarines are cut off from radio communication with their command authorities at ordinary radio frequencies. Submarines can surface and raise an antenna above the sea level, or float a tethered buoy carrying an antenna, then use ordinary radio transmissions; however, this makes them vulnerable to detection by anti-submarine warfare forces.

<span class="mw-page-title-main">Grimeton Radio Station</span> Historic Swedish wireless telegraphy station

Grimeton Radio Station in southern Sweden, close to Varberg in Halland, is an early longwave transatlantic wireless telegraphy station built in 1922–1924, that has been preserved as a historical site. From the 1920s through the 1940s it was used to transmit telegram traffic by Morse code to North America and other countries, and during World War II was Sweden's only telecommunication link with the rest of the world. It is the only remaining example of an early pre-electronic radio transmitter technology called an Alexanderson alternator. It was added to the UNESCO World Heritage List in 2004, with the statement: "Grimeton Radio Station, Varberg is an exceptionally well preserved example of a type of telecommunication centre, representing the technological achievements by the early 1920s, as well as documenting the further development over some three decades." The radio station is also an anchor site for the European Route of Industrial Heritage. The transmitter is still in operational condition, and each year on a day called Alexanderson Day is started up and transmits brief Morse code test transmissions, which can be received all over Europe.

<span class="mw-page-title-main">Umbrella antenna</span>

An umbrella antenna is a capacitively top-loaded wire monopole antenna, consisting in most cases of a mast fed at the ground end, to which a number of radial wires are connected at the top, sloping downwards. One side of the feedline supplying power from the transmitter is connected to the mast, and the other side to a ground (Earthing) system of radial wires buried in the earth under the antenna. They are used as transmitting antennas below 1 MHz, in the MF, LF and particularly the VLF bands, at frequencies sufficiently low that it is impractical or infeasible to build a full size quarter-wave monopole antenna. The outer end of each radial wire, sloping down from the top of the antenna, is connected by an insulator to a supporting rope or cable anchored to the ground; the radial wires can also support the mast as guy wires. The radial wires make the antenna look like the wire frame of a giant umbrella hence the name.

<span class="mw-page-title-main">Radio atmospheric signal</span> Broadband electromagnetic impulse

A radio atmospheric signal or sferic is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major attenuation in the Earth–ionosphere waveguide, and can be received thousands of kilometres from their source. On a time-domain plot, a sferic may appear as a single high-amplitude spike in the time-domain data. On a spectrogram, a sferic appears as a vertical stripe that may extend from a few kHz to several tens of kHz, depending on atmospheric conditions.

The Earth–ionosphere waveguide is the phenomenon in which certain radio waves can propagate in the space between the ground and the boundary of the ionosphere. Because the ionosphere contains charged particles, it can behave as a conductor. The earth operates as a ground plane, and the resulting cavity behaves as a large waveguide.

<span class="mw-page-title-main">VLF Transmitter Cutler</span> VLF radio transmitter operated by the US Navy

The VLF Transmitter Cutler is the United States Navy's very low frequency (VLF) shore radio station at Cutler, Maine. The station provides one-way communication to submarines of the Navy's Atlantic Fleet, both on the surface and submerged. It transmits with call sign NAA, at a frequency of 24 kHz and input power of up to 1.8 megawatts, and is one of the most powerful radio transmitters in the world.

James R. Wait was a Canadian electrical engineer and engineering physicist. In 1977, he was elected as a member of National Academy of Engineering in Electronics, Communication & Information Systems Engineering for his contributions to electromagnetic propagation engineering as it affects communication and geophysical exploration.

<span class="mw-page-title-main">Project Sanguine</span> US Navy research on communicating with submarines

Project Sanguine was a US Navy project proposed in 1968 for communication with submerged submarines using extremely low frequency (ELF) radio waves. The initially proposed system, hardened to survive a nuclear attack, would have required a giant antenna covering two-fifths of the state of Wisconsin. The proposed approach was never implemented because of protests and potential environmental impact. A smaller, less hardened system called Project ELF consisting of two linked ELF transmitters located at Clam Lake, Wisconsin 46°05′05.6″N90°55′03.7″W and Republic, Michigan 46°20′10.1″N87°53′04.6″W was built beginning in 1982 and operated from 1989 until 2004. The system transmitted at a frequency of 76 Hz. At ELF frequencies, the bandwidth of the transmission was very small, so the system could only send short-coded text messages at a very low data rate. These signals were used to summon specific vessels to the surface to receive longer operational orders by ordinary radio or satellite communication.

Naval Radio Transmitter Facility Aguada is a tall guyed radio mast erected by the United States Navy. It is used as a facility of the US Navy for ashore and U.S. and NATO ships, planes, and submarines operating at sea in areas of broadcast coverage near Aguada, Puerto Rico at 18°23′55″N67°10′38″W by using radio waves in the very low frequency range.

References

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  19. ↑ Cohen-Joppa, Felice (15 October 2004). "Project ELF Closes". The Nuclear Resistor. Felice and Jack Cohen-Joppa.
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