Counterpoise (ground system)

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Ground screen, similar to a counterpoise, at base of mast antenna of AM radio station KTBS Broadcast antenna counterpoise.jpg
Ground screen, similar to a counterpoise, at base of mast antenna of AM radio station KTBS
Diagram of counterpoise under the antenna mast of an AM radio station. It consists of a network of radial copper wires suspended above the ground, connected to the transmitter feedline ground. It is suspended about 8 feet above ground, so technicians can get access to the helix house at the foot of the tower. Counterpoise.jpg
Diagram of counterpoise under the antenna mast of an AM radio station. It consists of a network of radial copper wires suspended above the ground, connected to the transmitter feedline ground. It is suspended about 8 feet above ground, so technicians can get access to the helix house at the foot of the tower.
Antenna used in Lodge-Muirhead wireless system around 1900, the first counterpoise. Lodge-Muirhead antenna.png
Antenna used in Lodge-Muirhead wireless system around 1900, the first counterpoise.

In electronics and radio communication, a counterpoise is a network of suspended horizontal wires or cables (or a metal screen), used as a substitute for an earth (ground) connection in a radio antenna system. It is used with radio transmitters or receivers when a normal earth ground cannot be used because of high soil resistance [1] or when an antenna is mounted above ground level, for example, on a building. It usually consists of a single wire or network of horizontal wires, parallel to the ground, suspended above the ground under the antenna, connected to the receiver or transmitter's "ground" wire. [2] The counterpoise functions as one plate of a large capacitor, with the conductive layers of the earth acting as the other plate. [2] [3]

Contents

The counterpoise evolved with the Marconi (monopole) antenna during the 1890s, the first decade of radio in the wireless telegraphy era, but it was particularly advocated by British radio pioneer Oliver Lodge, [4] [5] and patented by his associate Alexander Muirhead [6] in 1907. [7]

Working principle

Counterpoises are typically used in antenna systems for radio transmitters where a good earth ground connection cannot be constructed.

Monopole antennas used at low frequencies, below 3 MHz, such as the mast radiator antennas used for AM broadcasting, require the radio transmitter to be electrically connected to the Earth under the antenna; this is called a ground (or earth). The ground serves as a capacitor plate to receive the displacement current from the antenna element and return it to the feedline from the transmitter. The ground connection must have a low electrical resistance, because it carries the full antenna current and any resistance in the ground connection will dissipate power from the transmitter. Low-resistance grounds for radio transmitters are normally constructed of a network of cables buried in the earth. However, in areas with dry, sandy or rocky soil the ground has a high resistance, so a low-resistance ground connection cannot be made. In these cases, a counterpoise is used. Another circumstance in which a counterpoise is used is when earth for a buried ground under the antenna mast is not available, such as in antennas located in a city or on top of a tall building.

A common design for a counterpoise is a series of radial wires suspended a few feet above the ground, extending from the base of the antenna in all directions in a "star" pattern, connected at the center. [2] The counterpoise functions as one plate of a large capacitor, with the conductive layers in the earth as the other plate. [2] Since the radio frequency alternating currents from the transmitter can pass through a capacitor, the counterpoise functions as a low-resistance ground connection. There should not be any closed loops in the wires of a counterpoise system, as the strong fields of the antenna will induce circular currents in it which will dissipate transmitter power.

Use at low frequencies

Inverted-L antenna with counterpoise, in a powerful amateur radio station, Colorado, 1920. The counterpoise is the lower grid of horizontal wires, suspended below the antenna. Inverted-L antenna with counterpoise.jpg
Inverted-L antenna with counterpoise, in a powerful amateur radio station, Colorado, 1920. The counterpoise is the lower grid of horizontal wires, suspended below the antenna.

The largest use of counterpoises is in transmitters on the low frequency (LF) and very low frequency (VLF) bands, as they are very sensitive to ground resistance. [2] Because of the large wavelength of the radio waves, feasible antennas used at these frequencies are electrically short, their length is small compared to the fundamental resonant length at the operating frequency, which is one-quarter of the wavelength. The radiation resistance of antennas (the resistance that represents power radiated as radio waves) drops as their length becomes small compared to a quarter wavelength, so the radiation resistance of antennas on the LF and VLF bands is very low, often as low as one ohm or less. The other, larger resistances in the antenna-ground circuit can consume significant portions of the transmitter’s power. The largest resistance in the antenna-ground circuit is often the ground system, and the transmitter power is divided proportionally between it and the radiation resistance, so the resistance of the ground system has to be kept very low to minimize the "wasted" transmitter power.

However, at low frequencies, the resistance of even a good ground system in high conductivity soil can consume a major portion of the transmitter power. Another source of resistance is dielectric losses from the penetration of radio waves into the ground near the antenna due to the large skin depth at low frequencies. Therefore, particularly at VLF frequencies, large counterpoises are sometimes used instead of buried grounds, to reduce the ground system resistance, allowing more of the transmitter power to be radiated.

Sometimes a counterpoise is combined with an ordinary ground, with the buried radial ground cables brought above ground near the base of the antenna to form a counterpoise. The area of the counterpoise around the base of the antenna is often covered with copper screening, to shield the ground to reduce ground currents.

Size

The size of the counterpoise used for radio work depends on the wavelength of the transmit frequency. With a monopole antenna, the counterpoise functions as a ground plane, reflecting the radio waves radiated downward by the antenna.[ citation needed ] To perform adequately, the counterpoise should extend at least half a wavelength from the antenna tower in all directions. [8] In designing a counterpoise for a medium-wave radio station, for example, radio-waves are a maximum of 566 metres (1,857 ft) long. Therefore, the counterpoise should extend 282 metres (925 ft) from the tower to make a circle 566 metres (1,857 ft) in diameter.

See also

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In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. The term has two different meanings in separate areas of electrical engineering. In antenna theory, a ground plane is a conducting surface large in comparison to the wavelength, such as the Earth, which is connected to the transmitter's ground wire and serves as a reflecting surface for radio waves. In printed circuit boards, a ground plane is a large area of copper foil on the board which is connected to the power supply ground terminal and serves as a return path for current from different components on the board.

Very low frequency 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.

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Medium frequency 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 hectometer. Frequencies immediately below MF are denoted 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.

Antenna (radio) 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.

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Whip antenna Type of radio antenna

A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so that it does not break easily, and the name is derived from the whip-like motion that it exhibits when disturbed. Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer whips, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 11 m long.

Wireless power transfer Transmission of electrical energy without wires as a physical link

Wireless power transfer (WPT), wireless power transmission, wireless energy transmission (WET), or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, a transmitter device, driven by electric power from a power source, generates a time-varying electromagnetic field, which transmits power across space to a receiver device, which extracts power from the field and supplies it to an electrical load. The technology of wireless power transmission can eliminate the use of the wires and batteries, thus increasing the mobility, convenience, and safety of an electronic device for all users. Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient, hazardous, or are not possible.

Spark-gap transmitter Type of radio transmitter

A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark. Spark-gap transmitters were the first type of radio transmitter, and were the main type used during the wireless telegraphy or "spark" era, the first three decades of radio, from 1887 to the end of World War I. German physicist Heinrich Hertz built the first experimental spark-gap transmitters in 1887, with which he proved the existence of radio waves and studied their properties.

Mast radiator Radio antenna consisting of a vertical mast in which the mast structure is energized and functions as the antenna

A mast radiator is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at low frequencies, in the LF and MF bands, in particular those used for AM radio broadcasting stations. The conductive steel mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna.

T-antenna Type of radio antenna

A T-antenna, T-aerial, flat-top antenna, top-hat antenna, or (capacitively) top-loaded antenna is a monopole radio antenna with transverse capacitive loading wires attached to its top. T-antennas are typically used in the VLF, LF, MF, and shortwave bands, and are widely used as transmitting antennas for amateur radio stations, and long wave and medium wave AM broadcasting stations. They can also be used as receiving antennas for shortwave listening.

Loop antenna Type of radio antenna

A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two distinct types:

Monopole antenna Type of radio antenna

A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane, which is often the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.

Folded unipole antenna Antenna used for radio broadcasts

The folded unipole antenna is a type of monopole antenna; it consists of a vertical metal rod or mast mounted over a conductive surface called a ground plane. The mast is surrounded by a "skirt" of vertical wires electrically attached at or near the top of the mast. The skirt wires are connected by a metal ring at the bottom and the feed line is connected between the ring and the ground.

Umbrella antenna

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. 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 (usually) insulated 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.

Ground dipole

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 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.

VLF Transmitter Cutler 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 in 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.

In antenna theory, radiation efficiency is a measure of how well a radio antenna converts the radio-frequency power accepted at its terminals into radiated power. Likewise, in a receiving antenna it describes the proportion of the radio wave's power intercepted by the antenna which is actually delivered as an electrical signal. It is not to be confused with antenna efficiency, which applies to aperture antennas such as a parabolic reflector or phased array, or antenna/aperture illumination efficiency, which relates the maximum directivity of an antenna/aperture to its standard directivity.

In radio systems, many different antenna types are used with specialized properties for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.

References

  1. ↑ Cebik, L. B. (December 31, 2009). "Counterpoise? On the Use and Abuse of a Word". antenneX. Archived from the original on December 19, 2016. Retrieved 25 September 2010. Alt URL
  2. 1 2 3 4 5 Laporte, Edmund (1952). Radio Antenna Engineering. MicGraw-Hill. pp. 52–53.
  3. ↑ United States Bureau of Naval Personnel (1973). Basic Electronics. Washington DC: Courier Corp. p. 523. ISBN   9780486210766.
  4. ↑ Lodge, Oliver (1925). Talks about Wireless. Cambridge University Press. pp. 91–92. ISBN   110805269X.
  5. ↑ Simmons, Harold H. (1908). Outlines of Electrical Engineering. New York: Cassell and Co. pp. 853–854.
  6. ↑ Alexander Muirhead, British patent no. 11271 "Hertzian Wireless Telegraphy"
  7. ↑ Eckersley, T. L. (May 1922). "An investigation of transmitting aerial resistances". Proc. Of the Inst. Of Electrical Engineers. London: E. and F. N. Spon. 60 (309): 599. Retrieved October 3, 2013.
  8. ↑ 20th edition of The ARRL Antenna Book in 2003, page 2-16
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