Counterpoise (ground system)
In
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.
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
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
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
See also
- Capacitance hat — the elevated counterpart of a ground system
- Monopole antenna
- Tesla coil
References
- ^ 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
- ^ a b c d e Laporte, Edmund (1952). Radio Antenna Engineering. MicGraw-Hill. pp. 52–53.
- ISBN 9780486210766.
- ISBN 110805269X.
- ^ Simmons, Harold H. (1908). Outlines of Electrical Engineering. New York: Cassell and Co. pp. 853–854.
- ^ Alexander Muirhead, British patent no. 11271 "Hertzian Wireless Telegraphy"
- ^ Eckersley, T. L. (May 1922). "An investigation of transmitting aerial resistances". Proc. Of the Inst. Of Electrical Engineers. 60 (309). London: E. and F. N. Spon: 599. Retrieved October 3, 2013.
- ^ 20th edition of The ARRL Antenna Book in 2003, page 2-16