Counterpoise (ground system)
A suspended wire network substituting for an earth ground in antenna systems.
A counterpoise is a substitute for a direct earth ground in a radio antenna system, made from a network of suspended horizontal wires, cables, or a metal screen. It is used with radio transmitters or receivers when a normal ground connection is impractical—for example, where soil resistance is high, or when the antenna is mounted above ground level, like on a building. The counterpoise acts as one plate of a large capacitor, with the conductive layers of the earth serving as the other plate.
This technique emerged with the Marconi (monopole) antenna in the 1890s, during the first decade of radio in the wireless telegraphy era, and was especially promoted by British radio pioneer Oliver Lodge, later patented by his associate Alexander Muirhead in 1907.
Counterpoises are most often used in antenna systems for radio transmitters where a good earth ground is not feasible. Monopole antennas operating below 3 MHz, such as the mast radiators used in AM broadcasting, require the transmitter to be electrically connected to the earth beneath the antenna. This ground connection acts as a capacitor plate, receiving displacement current from the antenna element and returning it to the transmitter feedline. The ground must have very low electrical resistance, as it carries the full antenna current; any resistance here wastes transmitter power. Normally, low-resistance grounds are built from buried cable networks, but in dry, sandy, or rocky soil, the ground resistance is too high. In such cases, a counterpoise is used. It is also employed when a buried ground under the antenna mast is unavailable, such as on city rooftops or tall buildings.
A common counterpoise design consists of radial wires suspended a few feet above the ground, extending from the antenna base in a star pattern and connected at the center. Because radio-frequency alternating currents can pass through a capacitor, the counterpoise provides a low-resistance ground connection. Closed loops in the counterpoise wires must be avoided, as the antenna’s strong fields can induce circular currents in them, wasting transmitter power.
The largest use of counterpoises is in low-frequency (LF) and very low-frequency (VLF) transmitters, which are highly sensitive to ground resistance.
- First advocate
- Oliver Lodge
- Patent holder
- Alexander Muirhead
- Patent year
- 1907
- Typical suspension height
- about 8 feet above ground
- Minimum recommended extension
- at least half a wavelength from the antenna tower
- Example wavelength for medium wave
- 566 m
- Example counterpoise diameter
- 566 m
Lore & Background
The counterpoise evolved with the Marconi (monopole) antenna during the 1890s, the first decade of radio in the wireless telegraphy era. It was particularly advocated by British radio pioneer Oliver Lodge and patented by his associate Alexander Muirhead in 1907. An early example was the antenna used in the Lodge-Muirhead wireless system around 1900, which is considered the first counterpoise.
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. There should not be any closed loops in the wires, as the strong fields of the antenna will induce circular currents in such loops, dissipating transmitter power. The counterpoise is often combined with an ordinary ground, with buried radial ground cables brought above ground near the base of the antenna to form a counterpoise; the area around the base may be covered with copper screening to shield the ground and reduce ground currents.
The largest use of counterpoises is in transmitters on the low frequency (LF) and very low frequency (VLF) bands, which are very sensitive to ground resistance. At these frequencies, the radiation resistance of antennas is very low—often one ohm or less—so the resistance of the ground system must be kept very low to minimize wasted transmitter power. Large counterpoises are sometimes used instead of buried grounds to reduce ground system resistance, allowing more transmitter power to be radiated.
Reader's Guide
The counterpoise holds particular significance for low-frequency and very-low-frequency radio transmitters, where ground resistance critically affects efficiency. Because antennas at these frequencies are electrically short, their radiation resistance is extremely low, and any resistance in the ground system consumes a major portion of transmitter power. The counterpoise provides a low-resistance path for displacement current, functioning as a capacitor plate with the earth, thereby reducing power loss. Its legacy is most evident in AM broadcasting and amateur radio installations where soil conditions or urban settings prevent effective buried grounds. The design principle—a network of radial wires suspended above ground—remains a standard solution for achieving a low-resistance ground connection without burial. The counterpoise also appears in historical contexts, such as the Lodge-Muirhead wireless system, marking its role in early radio development. While modern grounding techniques have advanced, the counterpoise continues to be used where traditional earth grounds are impractical, preserving its relevance in radio engineering.
Did You Know?
- A counterpoise should extend at least half a wavelength from the antenna tower in all directions to perform adequately.
- Closed loops in a counterpoise system can induce circular currents that dissipate transmitter power.
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