Counterpoise (ground system)

A counterpoise is a type of electrical ground that is not connected to earth. It is sometimes used to supplement a poor connection to an earth ground.[1] It consists of a network of wires or cables, or metal screen, running parallel to the ground, suspended a few centimeters to several meters above the ground, or sometimes 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 plate.

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 transmitter to be connected to a good ground under the antenna. This is usually accomplished with cables buried in the earth. However in areas with dry, sandy or rocky soil the ground has a high resistance, In these cases a counterpoise is used. The counterpoise functions as one plate of a capacitor with the conductive layers in the earth as the other plate. 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 extending from the base of the antenna in all directions, connected together at the center.

The size of the counterpoise used for radio work depends on the wavelength of the radio waves used. With a monopole antenna, the counterpoise functions as a ground plane, reflecting the radio waves radiated downward by the antenne. To perform adequately, the counterpoise should extend at least one-quarter of a 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, so the counterpoise should extend 464 feet (141 m) from the tower, for a circle 928 feet in diameter.

They are also used in high voltage systems, such as lightning rods and Tesla coils.

See also

Monopole antenna
Tesla coil

References

^ Cebik, L. B. (December 31, 2009). "Counterpoise? On the Use and Abuse of a Word". antenneX. Retrieved 25 September 2010.