Counterpoise (ground system)
Encyclopedia
A counterpoise is a type of electrical ground
that is not connected to earth. It is used in radio antenna
systems when a normal earth ground
cannot be used because of high soil resistance It consists of a network of wires or cables (or a metal screen) running parallel to the ground, suspended from a few centimetres to several metres above the ground (or lying on the surface). The counterpoise functions as one plate of a large capacitor
, with the conductive layers of the earth acting as the other.
where a good earth ground
is not available. Monopole antenna
s used at low frequencies, 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
. The ground must have a low electrical resistance, because 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; a low-resistance ground connection cannot be made. In these cases, a counterpoise is used instead. Since the radio frequency
alternating current
s from the transmitter can pass through a capacitor, the counterpoise functions as a low-resistance ground connection. A common design for a counterpoise is a series of radial wires close to the ground extending from the base of the antenna in all directions, connected together at the centre.
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. To perform adequately, the counterpoise should extend at least one-quarter wavelength from the antenna tower in all directions. In designing a counterpoise for an AM radio station
, for example, AM broadcast-band radio waves are a maximum of 1857 feet (566 m) long. Therefore, the counterpoise should extend 464 feet (141 m) from the tower to make a circle 928 feet (282.9 m) in diameter.
Counterpoises are also used in high-voltage systems, such as lightning rod
s and Tesla coils.
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
that is not connected to earth. It is used in radio antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
systems when a normal earth ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
cannot be used because of high soil resistance It consists of a network of wires or cables (or a metal screen) running parallel to the ground, suspended from a few centimetres to several metres above the ground (or lying on the surface). The counterpoise functions as one plate of a large capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, with the conductive layers of the earth acting as the other.
Uses
Counterpoises are typically used in antenna systemsAntenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
where a good earth ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
is not available. Monopole antenna
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 driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...
s used at low frequencies, such as the mast radiator
Mast radiator
A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...
antennas used for AM broadcasting
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
, require the radio transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
to be electrically connected to the earth under the antenna; this is called a ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
. The ground must have a low electrical resistance, because 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; a low-resistance ground connection cannot be made. In these cases, a counterpoise is used instead. Since the radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
s from the transmitter can pass through a capacitor, the counterpoise functions as a low-resistance ground connection. A common design for a counterpoise is a series of radial wires close to the ground extending from the base of the antenna in all directions, connected together at the centre.
The size of the counterpoise used for radio work depends on the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the transmit frequency. With a monopole antenna the counterpoise functions as a ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
, reflecting the radio waves radiated downward by the antenna. To perform adequately, the counterpoise should extend at least one-quarter wavelength from the antenna tower in all directions. In designing a counterpoise for an AM radio station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
, for example, AM broadcast-band radio waves are a maximum of 1857 feet (566 m) long. Therefore, the counterpoise should extend 464 feet (141 m) from the tower to make a circle 928 feet (282.9 m) in diameter.
Counterpoises are also used in high-voltage systems, such as lightning rod
Lightning rod
A lightning rod or lightning conductor is a metal rod or conductor mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning...
s and Tesla coils.