Antenna Technology

Counterpoise

Q: How many radials does a counterpoise need to work well?

The radial count trades ground loss against installation effort. For an elevated counterpoise, 2 to 4 quarter-wave radials per band are usually enough because the resonant radials carry most of the return current and there is no soil contact loss; field measurements show diminishing returns beyond about 4 elevated radials. For an on-ground or buried radial system the picture is different: ground loss falls steadily as radials are added, and the classic Brown, Lewis and Epstein study found that roughly 120 radials each about 0.4 wavelength long are needed to approach the performance of a perfect ground. A practical compromise of 30 to 60 buried radials typically recovers most of the efficiency a broadcast vertical can achieve.

Q: What is the difference between a counterpoise and a ground plane?

Both provide the image that a monopole needs, but they differ in form and electrical behavior. A ground plane is a continuous or near-continuous conductive sheet, or a dense set of radials, that approximates an infinite perfect conductor and forces a low, predictable feedpoint impedance near 36 ohms for a quarter-wave element. A counterpoise is a sparse, often resonant network of wires that is deliberately tuned; its limited size makes the feedpoint impedance and pattern depend on radial length, height above earth, and the number of radials. In short, a ground plane aims to mimic perfect earth, while a counterpoise is an engineered, frequency-dependent substitute used where a real ground connection is impractical.

Q: How long should counterpoise radials be at a given frequency?

Elevated radials are normally cut to a physical length near a quarter wavelength times a velocity factor of about 0.95 to 0.97 for insulated wire, so a single-band elevated counterpoise for the 14 MHz band uses radials near 5.0 to 5.2 meters. Because each elevated radial is itself a resonant element, its length sets the resonant frequency and detunes the antenna if it is wrong. Buried or on-ground radials behave differently: they are not resonant, so length is chosen to cover the lossy near field rather than to resonate, and 0.2 to 0.4 wavelength is typical. Multiband counterpoises use separate resonant radials for each band or a single longer wire trimmed for acceptable VSWR across the range.

/KOWN-ter-poyz/

Acting as an artificial RF ground, a counterpoise is a network of conductors, most often a set of elevated radial wires, that substitutes for a direct earth connection and completes the return path for an unbalanced antenna. A quarter-wave monopole radiates efficiently only when it works against the lower half of its current distribution; where a buried ground plane is impractical, a counterpoise supplies that missing half through image theory. Because the radials are typically resonant and few in number, their length, height, and count strongly set the feedpoint impedance and the elevation pattern. A well-built counterpoise can cut ground loss from several ohms to a fraction of an ohm, raising radiation efficiency on the 1.8 to 30 MHz bands where soil conductivity would otherwise dominate the loss budget.

Category: Antenna Technology

Elevated radials: 2 to 4 per band

Radial length: ≈ 0.25λ × VF

How a Counterpoise Completes the Antenna

A vertical or monopole element is only half of a working antenna. The driven element handles one polarity of the standing-wave current, and the other polarity must flow into some reference structure so the feedline sees a complete circuit. In a textbook installation that reference is a large conducting earth or a dense radial field that behaves like a perfect mirror, placing an electrical image of the element below the boundary. When the antenna sits on a rooftop, a tower platform, a vehicle, or any site where real earth is too far away or too lossy, a counterpoise stands in for that mirror as a deliberately engineered set of wires raised above the ground.

The defining trait of a counterpoise is that it is sparse and usually resonant rather than continuous. Two to four radials cut near a quarter wavelength carry the bulk of the return current, and because they are elevated they avoid direct contact with lossy soil. That isolation is what lets a handful of elevated radials rival a system of dozens of buried wires: the displacement current that would otherwise leak into the dielectric earth is instead confined to the air between the radials and the ground below. The penalty is sensitivity. Trimming a radial, changing its height, or adding asymmetry shifts the resonant frequency and tilts the pattern, so a counterpoise must be tuned as carefully as the driven element itself.

Electrically, the counterpoise sets the bottom of the feedpoint impedance. A quarter-wave monopole over a perfect ground presents about 36 ohms; over a real or imperfect counterpoise the resistive part rises by the ground-loss resistance, and the reactive part swings with radial length. Designers minimize that loss term because it dissipates power as heat instead of radiating it, which is the entire reason the counterpoise exists.

Elevated Radials Versus Buried Radial Fields

The two common implementations behave very differently. An elevated counterpoise relies on a small number of resonant radials and benefits from height that lifts the fields away from the soil. A buried or on-ground radial field is not resonant at all; its job is to intercept the lossy near-field return current spread across the ground, so efficiency climbs steadily as more radials are added rather than depending on precise tuning. Broadcast practice settled on roughly 120 buried radials about 0.4 wavelength long, while a rooftop amateur vertical often performs as well with just four elevated radials.

Counterpoise Sizing Equations

Resonant radial length (elevated):
L_radial ≈ (75 / f_MHz) × VF meters (VF ≈ 0.95 to 0.97)

Feedpoint resistance with ground loss:
R_in = R_rad + R_ground (quarter-wave monopole: R_rad ≈ 36 Ω)

Radiation efficiency:
η = R_rad / (R_rad + R_ground) × 100%

Where f_MHz = operating frequency, VF = wire velocity factor, R_rad = radiation resistance, R_ground = ground-system loss resistance. Example: at 14.2 MHz an insulated elevated radial ≈ (75 / 14.2) × 0.96 ≈ 5.07 m; if R_ground drops from 6 Ω to 0.5 Ω, η rises from 86% to 99%.

Ground-System Comparison

Ground system	Conductor count	Typical length	Resonant?	Ground loss	Best use
Elevated counterpoise	2 to 4 per band	0.25λ × VF	Yes	0.3 to 2 Ω	Rooftop / tower verticals
Buried radial field	30 to 120	0.2 to 0.4λ	No	1 to 5 Ω	Broadcast monopoles
Solid ground plane	Continuous sheet	≥ 0.5λ radius	No	< 0.3 Ω	VHF/UHF whips, test ranges
Single radial / tiger tail	1	0.25λ	Yes	3 to 8 Ω	Handheld and portable whips
Earth stake only	1 rod	1 to 3 m	No	10 to 100+ Ω	Safety/DC ground, not RF

Common Questions

Frequently Asked Questions

How many radials does a counterpoise need to work well?

It depends on the type. An elevated counterpoise usually needs only 2 to 4 resonant radials per band because they carry most of the return current with no soil-contact loss, and field tests show diminishing returns beyond about four. A buried field is different: ground loss falls as radials are added, and the Brown, Lewis and Epstein work showed roughly 120 radials near 0.4λ approach a perfect ground. A practical 30 to 60 buried radials recovers most of the achievable efficiency.

What is the difference between a counterpoise and a ground plane?

A ground plane is a continuous sheet or dense radial set that mimics an infinite perfect conductor, forcing a low, predictable feedpoint near 36 Ω for a quarter-wave element. A counterpoise is a sparse, usually resonant network of wires that is deliberately tuned; its limited size makes the impedance and pattern depend on radial length, height, and count. The ground plane chases perfect earth, while the counterpoise is an engineered, frequency-dependent substitute for it.

How long should counterpoise radials be at a given frequency?

Elevated radials are cut near a quarter wavelength times a velocity factor of about 0.95 to 0.97 for insulated wire, so a 14 MHz elevated radial runs about 5.0 to 5.2 m. Each elevated radial is resonant, so wrong length detunes the antenna. Buried or on-ground radials are not resonant; length is chosen to cover the lossy near field, with 0.2 to 0.4λ typical. Multiband designs use separate resonant radials per band or one longer wire trimmed for acceptable VSWR.

Related Terms

← Counterfeit Parts Coupled-Cavity TWT →