Ground Tuning Units: What an Artificial RF Ground Can—and Cannot—Do

“Artificial ground” is a product-category name, not a new kind of earth. A typical unit places an adjustable reactance network and an RF-current indicator between a station chassis node and a counterpoise wire or existing ground lead. Tuning can make that branch easier to drive at one frequency. Whether the change improves radiation, merely moves current, or increases loss depends on the entire antenna system.

1. Four Different Jobs Are Commonly Called “Ground”

Confusion begins when one word is used for conductors with different purposes. These functions can meet at prescribed bonding points, but they are not interchangeable.

A counterpoise can be intentionally insulated from soil. A safety bond can carry little current during normal operation yet be essential during a fault. A radial system can reduce antenna ground loss without being the building’s PE. Calling all three “ground” does not give them the same electrical job.

2. What the Tuning Network Actually Changes

At one frequency, the impedance looking into a counterpoise or ground-lead branch can be represented at the GTU terminals as:

An adjustable network contributes its own reactance and, in a real unit, some loss. A useful tuning condition may bring the net reactive part near zero:

That does not make the branch impedance zero. The remaining real part can represent conductor and network loss, soil loss, and power radiated by the counterpoise, lead, chassis or attached structure. Those terms cannot be separated by an SWR reading or by the GTU ammeter.

The official MFJ-931 instruction manual documents a specific operating procedure: connect the unit between the transmitter or tuner chassis and a counterpoise or ground lead, adjust for maximum indicated branch current, then re-adjust the antenna tuner. It also warns that the product does not provide a DC electrical ground and that the open counterpoise end can cause an RF burn.

The claim that a tuner “places a far-away RF ground at the radio” is best treated as operational shorthand. A matching network can cancel input reactance at its own terminals. It cannot make every point along a long wire share one RF potential; the wire still has distributed current, voltage and fields.

3. The Meter Sees One Branch of a Larger Network

A real station rarely has only the path drawn on a product sketch. Its chassis node may also connect to:

• the outside of a coaxial feedline;
• a protective conductor through Class I equipment or a power supply;
• USB, audio, control and microphone cable shields;
• a mast, mounting frame, vehicle body or balcony metalwork;
• surge-protection and prescribed bonding conductors; and
• the operator and nearby objects through capacitance.

RF current divides among all available paths according to their coupled impedances. Adding or tuning one branch changes the whole solution, so the antenna tuner and GTU may interact. A GTU ammeter reports only the current through its own branch. A low reading may mean another conductor is carrying the current; a high reading may be intended counterpoise current, lossy current, radiating current, or a mixture.

                           coax exterior ── feedpoint / mast / environment
Transmitter chassis node ─ protective earth and bonded equipment
                           control, audio and computer cable shields
                           GTU ── counterpoise or long ground lead

The MFJ manual itself notes that a parallel ground connection can bypass its meter branch. That is a circuit observation, not permission to remove a safety conductor or required bond. Safety connections stay in place; the RF system must be designed around them.

4. A Station-Side GTU Does Not Automatically Replace Feedpoint Radials

Topology matters more than the product label.

For a coax-fed monopole, the intended return structure belongs at the feedpoint. If the coax exterior is allowed to complete the antenna instead, its current distribution depends on feedline length, routing, station bonds and nearby objects. A station-side tuner can alter that distribution, but it has not moved a missing radial field from the antenna base into the shack.

5. Why Physical Radials Still Matter

A vertical over real earth is not simply “half a dipole” with soil filling in the missing arm. The image model is exact only for an ideal infinite conducting plane. With finite conductivity and a finite radial system, earth fields, conductor currents, loss and pattern must be solved together.

Brown, Lewis and Epstein’s 1937 ground-system experiments measured feedpoint impedance, radial and earth currents, and field intensity for different vertical and buried-radial configurations. The work demonstrated why physical ground-system geometry affects efficiency; it did not establish that a reactive tuner can replace copper near the base.

Rudy Severns, N6LF, later tested amateur-scale systems with tighter control. His 7.2 MHz surface-versus-elevated-radial experiment recorded feedpoint impedance and relative S21, used a feedline common-mode choke, measured current division and repeated the sequence on different days. That method matters: radial performance was supported by multiple measurements while unwanted feedline current was deliberately controlled.

A tuned short counterpoise can be a reasonable compromise where many radials are impossible. Resonance can reduce the reactive burden presented at the selected port. It does not guarantee low loss, stable pattern, balanced current or broadband behaviour.

6. Return Current and Common-Mode Current Are Not Synonyms

In the wanted differential mode of a coaxial line, current on the centre conductor is accompanied by equal and opposite current on the inner surface of the outer conductor. Additional current on the outer surface of the shield flows with an external return through the surroundings. That exterior component is the common-mode current of the coax-plus-environment circuit.

This modal definition is more precise than calling every current that takes an unexpected path “common mode.” ITU-R’s two-conductor decomposition expresses conductor currents as differential and common-mode components and shows why imbalance converts one mode into the other.

Current in a GTU counterpoise branch may be the intended antenna return current. Current on a coax exterior may be common mode. Current in a PE or bonding conductor is another identifiable branch. Naming and measuring each conductor is more useful than assigning every stray ampere one label.

A common-mode choke raises the impedance of the coax-exterior path while ideally leaving the wanted coax mode substantially unchanged. It can redirect current into the intended counterpoise, but its result depends on placement and the impedance of every alternative path. It cannot replace a counterpoise, PE, a required bond or lightning protection.

7. Protective Earth and Lightning Protection Come First

IEC 60364-5-54:2011+AMD1:2021 addresses earthing arrangements, protective conductors and protective bonding conductors for electrical-installation safety. For Belgium, use the current official AREI/RGIE books and competent local design or inspection advice. A product described as an “RF ground” does not modify those requirements.

ITU-T K.71, currently in force for customer antenna installations within its scope, separates mains-contact protection, bonding, surge protection and lightning-protection-system decisions. It requires prescribed conductors and coordinated connection points when its installation cases call for them. A variable RF network is not one of those protective conductors.

Do not add an isolated “RF earth” rod to a building as though it were independent of the electrical and lightning system. An electrode or structural bond must be assessed and integrated under the applicable rules. Do not use water pipes, gas pipes, balcony rails or random building steel as a counterpoise merely because they are metal.

8. The Counterpoise Is an Accessible Antenna Conductor

A tuned counterpoise can carry substantial RF current and can develop high voltage, especially near an open end or a high-impedance point. The counterpoise, GTU terminals, clips, frame and any attached plate must therefore be treated as active parts of the transmitting antenna—not as harmless “ground.”

The ICNIRP 2020 RF guidelines explain that contact current between a person and an energized conductive object can cause pain and potentially tissue damage. The result depends on frequency, contact area, posture and environment, so ICNIRP gives risk-management guidance rather than a single universal contact-current limit.

• Tune with the lowest transmitter power that gives a stable indication.
• Enclose or guard the GTU terminals and use insulation rated for the expected RF voltage.
• Keep counterpoises, open ends and attached metal away from people, animals and public access.
• Do not make a bicycle frame, trolley, vehicle body, balcony rail or “ground foot” RF-hot where it can be touched.
• Reassess the arrangement after any change in frequency, power, wire routing, weather or nearby objects.

Tape over a wire end, as the manufacturer manual recommends, is a basic precaution; it is not by itself a complete barrier, enclosure, clearance plan or exposure assessment.

9. Salt Water Helps Only When the Geometry Couples to It

Sea water can be a favourable RF environment because its electrical properties differ greatly from ordinary soil. ITU-R P.527-6 models the frequency-, temperature- and salinity-dependent complex permittivity and conductivity of sea water and other surfaces.

That fact does not prove that a small insulated plate, shoe, cart or wire couples efficiently into the sea. Capacitance, area, height, insulation, distance to water and the rest of the current path all matter. Wet ground or salt water also does not make accessible metal safe to touch during transmission.

10. A Defensible Tuning and Test Sequence

1. Draw the real circuit. Include the radiator, feedpoint return, coax exterior, PE, bonds, mast, control cables and every intentional counterpoise.
2. Preserve the safety architecture. Do not alter required PE, equipotential bonding, surge protection, lightning conductors or clearances.
3. Choose the branch deliberately. Place the GTU only in an intended RF counterpoise or ground-lead branch, never in a protective conductor.
4. Make conductors inaccessible. Guard terminals and open ends before applying RF.
5. Tune at low power. Follow the specific unit’s manual. For the MFJ-931 topology, peak its branch-current indication and then re-adjust the antenna tuner; expect interaction.
6. Measure other paths. Check coax-exterior and cable currents at multiple positions. A lower GTU indication may simply mean that current moved elsewhere.
7. Compare at constant accepted power. Record feedpoint impedance, network loss, component heating and a repeatable remote field or received-signal metric. A nearby field-strength meter can be dominated by reactive fields and pattern changes.
8. Repeat after environmental changes. Moving a wire, operator, cart or cable can change the tuning and current division.

11. Takeaways You Can Trust

• An artificial RF ground is a tuned conductor branch, not ideal earth.
• Reactive cancellation changes input impedance; it does not erase resistance, radiation or distributed voltage.
• The built-in meter measures only current through the GTU branch.
• A station-side unit does not automatically replace radials at a remote antenna feedpoint.
• Physical radial and counterpoise geometry still controls fields, loss, balance and pattern.
• Intended return current and coax common-mode current must be identified separately.
• A choke can redirect RF current but cannot replace PE, prescribed bonding or lightning protection.
• Counterpoises and attached metal are active antenna conductors with possible RF contact hazards.
• Salt water is conductive, but efficient and safe coupling still depends on geometry.
• Judge the installation with branch-current measurements, controlled remote-field evidence, heat, repeatability and safety—not SWR alone.

Primary Sources and Scope Anchors

• MFJ-931 instruction manual—the manufacturer’s connection, tuning and RF-burn warnings; not independent proof of efficiency.
• Brown, Lewis and Epstein, “Ground Systems as a Factor in Antenna Efficiency,” Proceedings of the IRE, June 1937—radial, earth-current, impedance and field measurements.
• N6LF, Experimental Determination of Ground System Performance for HF Verticals, Part 3—controlled surface/elevated-radial measurements with feedline isolation and current-division checks.
• ITU-R Report SM.2158-2—differential/common-mode current decomposition and mode conversion.
• ITU-T K.71, IEC 60364-5-54:2011+AMD1:2021 and the official Belgian AREI/RGIE—installation-safety, earthing and bonding scope anchors.
• ICNIRP 2020 RF guidelines—RF exposure, limb-current reference levels and contact-current guidance.
• ITU-R P.527-6—electrical characteristics of sea water, soil and other Earth surfaces.

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