- Counterpoise (ground system)
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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[1] 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.
Contents
Uses
Counterpoises are typically used in antenna systems where a good earth ground is not available. Monopole antennas 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 currents 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 (283 m) in diameter. Counterpoises are also used in high-voltage systems, such as lightning rods and Tesla coils.
See also
References
- ^ Cebik, L. B. (December 31, 2009). "Counterpoise? On the Use and Abuse of a Word". antenneX. http://www.antennex.com/shack/Dec06/cps.html. Retrieved 25 September 2010.
External links
Categories:- Antennas (radio)
- Radio electronics
- Lightning
- Electric arcs
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