electronics Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other Electric charge, electrically charged particles. It is a subfield ...

and

radio communications Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected t ...

, a counterpoise is a network of suspended horizontal wires or cables (or a metal screen), used as a substitute for an

earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

( 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 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. The counterpoise functions as one plate of a large

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

, with the

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...

layers of the earth acting as the other plate. The counterpoise evolved with the Marconi (monopole) antenna during the 1890s, the first decade of radio in the

wireless telegraphy Wireless telegraphy or radiotelegraphy is the transmission of text messages by radio waves, analogous to electrical telegraphy using electrical cable, cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimenta ...

era, but it was particularly advocated by British radio pioneer

Oliver Lodge Sir Oliver Joseph Lodge (12 June 1851 – 22 August 1940) was an English physicist whose investigations into electromagnetic radiation contributed to the development of Radio, radio communication. He identified electromagnetic radiation indepe ...

, and patented by his associate Alexander MuirheadAlexander Muirhead, British patent no. 11271 "Hertzian Wireless Telegraphy" in 1907.

Working principle

Counterpoises are typically used in antenna systems for radio

transmitter In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...

s where a good earth ground connection cannot be constructed.

Monopole 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 current from the transmitter is applied, or for rece ...

s used at low frequencies, below 3

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base u ...

, such as the

mast radiator A mast radiator (or radiating tower) 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 l ...

antennas used for

AM broadcasting AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transm ...

, require the

radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna with the purpose of signal transmissio ...

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 In electromagnetism, displacement current density is the quantity appearing in Maxwell's equations that is defined in terms of the rate of change of , the electric displacement field. Displacement current density has the same units as electric ...

from the antenna element and return it to the feedline from the transmitter. The ground connection must have a low

electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...

, 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. The counterpoise functions as one plate of a large

, with the conductive layers in the earth as the other plate. Since the

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...

alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...

s 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

The largest use of counterpoises is in transmitters on the

low frequency 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. LF radio waves ...

(LF) and

very low frequency 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 ...

(VLF) bands, as they are very sensitive to ground resistance. Because of the large

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

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

. The

radiation resistance Radiation resistance is that part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna. A radio transmitter applies a radio frequency alternating current to an antenna, which radiates the energy ...

of antennas (the resistance that represents power radiated as radio waves) drops as their length becomes small compared to a quarter wavelength, so the

of antennas on the LF and VLF bands is very low, often as low as one

ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (1 ...

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 In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...

losses from the penetration of radio waves into the ground near the antenna due to the large

skin depth In electromagnetism, skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with gre ...

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

of the transmit frequency. With a monopole antenna, the counterpoise functions as a

ground plane In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. Ground planes are typically made of copper or aluminum, and they are often located on the bottom of printed circuit boards ...

, effectively inducing the radio wave currents to form a reflection of the antenna. To perform adequately, the counterpoise should extend at least half a wavelength from the antenna tower in all directions.20th edition of The ARRL Antenna Book in 2003, page 2-16 In designing a counterpoise for a medium-wave

radio station Radio broadcasting is the broadcasting of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based rad ...

, for example, radio-waves are a maximum of long. Therefore, the counterpoise should extend from the tower to make a circle in diameter.

References

{{Reflist , refs= {{cite web , url = http://www.antennex.com/shack/Dec06/cps.html , title = Counterpoise? On the Use and Abuse of a Word , first = L. B. , last = Cebik , authorlink = , date = December 31, 2009 , work = antenneX , publisher = , archiveurl = https://web.archive.org/web/20161219005119/http://www.antennex.com/shack/Dec06/cps.html , archivedate = December 19, 2016 , accessdate = 25 September 2010 }
Alt URL

External links

Papalexopoulos, A. D., Meliopoulos, A. P., "Frequency Dependent Characteristics of Grounding Systems". ''Power Delivery, IEEE Transactions'', vol. 2 issue 4 (Oct. 1987), pp. 1073–1081
Antennas (radio) History of air traffic control Radio electronics Lightning Electric arcs

Working principle

Use at low frequencies

Size

See also

References

External links