Ground Mirrors and Radials: Not All “Grounds” Are Equal
Ground Mirrors and Radials: Not All “Grounds” Are Equal
Soil, radial wires, counterpoises, metal roofs and protective earth can all appear in one station drawing. They do not perform the same electrical job, and none deserves a fixed RF verdict before the installed current is measured.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
When somebody tells me an antenna is “grounded,” I still do not know where its RF current returns. The word may refer to dirt, an electrode, a radial field, a metal roof, a feed-line shield or protective earth. The useful questions are where current flows, what is intentional, what is lossy and what is required for safety.
My practical position: build a deliberate return structure and judge it as part of the complete antenna. Do not replace the old “more copper is always better” slogan with a new “many very short radials always win” slogan. Length, count, spacing, soil, elevation, symmetry, total wire and the other conductors at the site interact.
Real Earth Is Neither a Perfect Mirror nor Always the Enemy
Earth is a conductor and a dielectric with finite, frequency-dependent properties. Current can flow through it and fields can couple into it. The resulting loss and pattern depend on conductivity, permittivity, moisture, temperature, layering, topography and frequency. ITU-R P.527 exists precisely because “average ground” is not one universal material.
Calling soil the enemy is too simple. It is part of the electromagnetic environment whether we like it or not. For a ground-mounted monopole, forcing strong return current through lossy soil near the feedpoint can waste power, and deliberate conductors can reduce that loss. Farther from the antenna, ground properties also help determine propagation and the elevation pattern. A site with conductive soil may support a different result from dry rock, but neither a low SWR nor a damp patch proves low ground loss.
The honest model therefore includes real earth rather than pretending it is an ideal zero-volt plane. It also avoids blaming every seasonal impedance change on soil: vegetation, water on insulators, cable position, nearby metal and connection condition can move at the same time.
An Earth Rod Can Join the RF Network Without Becoming an RF Sink
An electrode bonded to a station or mast can carry RF current if the installation provides a path. That does not make it an infinite reservoir into which current disappears. The rod, bonding conductor, soil and every connected service have distributed resistance, inductance and capacitance. At HF, a physically modest conductor can already be a significant electrical length.
This distinction matters in both directions. An earth electrode required by the electrical or lightning-protection design must not be removed because RF current is found on it. At the same time, adding an isolated rod is not a guaranteed cure for RF feedback, noise or SWR. It may create another current branch, change common-mode current or change loss without solving the original mechanism.
Protective earthing, equipotential bonding and lightning protection are governed by their safety purpose and applicable rules. IEC 60364-5-54 addresses earthing arrangements and protective conductors; IEC 62305 addresses lightning protection as a coordinated system. Radials and counterpoises do not replace either function.
Radials Are Engineered Return Conductors
A monopole needs a return branch. Ground screens and radial wires are ways to put more of that current on known conductors and less through poorly controlled lossy paths. They can reduce ground loss, change feedpoint impedance, redistribute current from the feed-line exterior and make the installation more repeatable. They are not merely decorative “image wires.”
ITU-R BS.705-2 treats monopole height, earth-system radius, radial number and conductor diameter as explicit model variables. That is a better discipline than a universal radial count. The relevant design record includes:
- radiator height, loading and feedpoint position;
- frequency or band set;
- radial length, count, spacing, conductor size and total wire;
- whether wires are buried, on the surface or elevated;
- soil electrical properties and recent moisture;
- mast, fence, roof, feed-line shield, bonding and control-cable paths.
The first intentional conductors can produce a large change when the baseline is a poorly controlled return path. Further conductors often produce smaller changes, but “diminishing returns” does not identify a fixed optimum. The curve moves with radiator geometry, radial length, soil and the comparison constraint. Eight long wires, sixteen short wires and the same total copper divided another way are three different experiments.
What Rudy Severns Actually Gives Us
Rudy Severns, N6LF, did the valuable thing: he built controlled systems and measured them. His work does not support a fashionable rule that many very short radials are always the best use of copper. It shows why every result must keep its radiator, band, soil, radial geometry and measurement method attached.
In his QEX ground-system series, Part 3 compared one specific surface system of sixty-four radials with one carefully adjusted four-radial system elevated 48 inches, at 7.2 MHz. Their measured transmission difference was only 0.1 dB in that experiment. That is evidence for those installations, not proof that four elevated radials universally equal sixty-four surface radials.
His later elevated-ground-system work investigated current asymmetry and the sensitivity of sparse radial fans to nearby conductors, soil and unequal geometry. Adding more elevated radials can improve robustness because one disturbed wire carries a smaller fraction of the total current. That does not produce a universal elevated-radial count or guarantee a symmetric pattern at another site.
Part 6 of the 2009 series compared specific multiband sets and uniform-length sets. Some shorter or mixed arrangements used copper effectively within those tests; some long sparse arrangements behaved badly on particular bands because their electrical lengths interacted with the system. The conclusion is not “shorter always wins.” It is that multiband current distribution can defeat a radial recipe that looks reasonable from wire length alone.
The N6LF lesson I keep: controlled measurements expose diminishing returns and resonance effects, but they do not erase geometry. Start with a return structure appropriate to the lowest and most important bands, then measure before trading useful reach or symmetry for more visual wire count.
Elevated and On-Ground Radials Are Different Antennas
On-ground or shallow-buried radials couple strongly to soil and act as a distributed screen. Elevated radials are more recognisably part of the radiating structure. Their electrical length, height, symmetry, tuning and coupling to the vertical determine current division and pattern.
A few well-symmetrised elevated radials can have low conductor and ground loss in a suitable installation. A broad surface field can be mechanically robust and less sensitive to one nearby object. Either can outperform the other under selected conditions. Neither is universally superior.
The comparison becomes especially misleading if “height wins” actually means that the complete feedpoint and radiator were raised. Raising only the return conductors changes a different set of couplings from raising the entire antenna. Record both heights and keep accepted power and measurement geometry fixed.
Elevated conductors can carry significant RF voltage and current. They need mechanical integrity, clearance and access control appropriate to the installation and operating power. That RF hazard does not turn them into protective earth.
A Counterpoise Is a Named Return Conductor, Not a Magic Ground
“Counterpoise” is used for several architectures: a tuned elevated wire, a radial fan, a capacitively coupled conductor or simply the visible portion of an end-fed antenna’s return side. The name alone says nothing about its impedance or how much current remains on the coax exterior.
Trace the complete path through the matching network, intentional counterpoise, mast, capacitance to surroundings, feed-line shield and station wiring. A conductor need not touch soil to carry return current. A conductor that does touch soil may still be only one branch in a distributed network.
If impedance transformation and common-mode suppression are both needed, specify them as separate functions. A transformer selected for the measured load does not automatically provide enough common-mode impedance, and a choke does not supply the missing antenna return. Choke placement follows the unwanted current path and the boundary we want to establish; there is no fixed distance that works for every system.
Metal Roofs and Structures Are Geometry-Dependent Conductors
A conductive roof, container, gutter, fence, solar-panel frame or mast can carry induced or directly connected current. Calling it an RF mirror is a useful first picture only when its dimensions, continuity and location support the approximation. Real structures have seams, coatings, bonds, cables and irregular edges. They can behave as a ground plane, a counterpoise, a parasitic element, part of the feed or several of those at once.
A metal roof can reduce return loss in one installation and distort the pattern or couple RF into building wiring in another. Its size relative to wavelength, connection geometry, roof pitch, height, material continuity and surrounding services matter. Even an electrically continuous roof need not produce a symmetric current distribution.
Do not assume that a solar-panel frame or gutter is safe to use as an RF conductor. Existing electrical bonding, equipment instructions, corrosion, touch voltage, lightning protection and building rules remain controlling. Treat an unplanned structure as part of the antenna model until measurements show otherwise—not as free ground.
Protective Earth Must Stay Protective
Protective earth is intended to support protection against electric shock and fault clearing. It is not installed as an antenna-efficiency component. If RF current flows on it, diagnose why the antenna system is coupling into the station wiring; do not lift the protective conductor.
Likewise, a required bond or lightning conductor cannot be replaced by a resonant wire, a radial fan or a choke. Fast impulse current sees conductor routing and inductance very differently from HF differential current. IEC 62305-1 treats lightning protection at system level because a lone rod, protector or disconnect is not the complete answer.
A well-integrated station can use protective bonding, lightning protection and RF current control together. Their conductors may meet, but their design criteria remain distinct.
Measure the Ground System You Actually Built
Feedpoint SWR is not enough. A lower value can result from lower loss, higher loss or a changed transformation. Use measurements that can separate current distribution, accepted power and pattern:
- Declare the reference plane. Save complex impedance at the feedpoint and at the shack with the feed-line loss and electrical length recorded.
- Map current. Compare individual radial currents, mast current and coax-exterior current at repeatable positions.
- Hold accepted power constant. Do not call a remote field change an efficiency result when mismatch or matching loss changed.
- Measure more than one direction. A roof, fence or asymmetric fan can move the pattern rather than changing total radiated power.
- Use A/B/A restoration. Restore the baseline and record soil moisture, weather, cable positions and connection condition.
- Inspect stress and safety. Check joint heating, corrosion, RF voltage, accessibility and every required bond without creating an unsafe test state.
The most useful result is not “this many radials is best.” It is a current and field record tied to a declared installation. That record can guide the next metre of copper far better than a slogan.
Primary Engineering Sources
- ITU-R P.527-6 — Electrical characteristics of the surface of the Earth: frequency-dependent soil permittivity and conductivity.
- ITU-R BS.705-2 — HF transmitting and receiving antennas: vertical-monopole and earth-system geometry as explicit model inputs.
- Rudy Severns, N6LF — Ground System Performance for HF Verticals, Part 1: test range, instrumentation and limits of the experimental series.
- Rudy Severns, N6LF — Ground System Performance for HF Verticals, Part 3: the bounded surface-versus-elevated comparison.
- Rudy Severns, N6LF — Ground Systems for Multiband Verticals: measured multiband and uniform-length radial configurations.
- Rudy Severns, N6LF — Experimental Determination of Ground-System Performance for HF Verticals, Part 1: elevated-system sensitivity, symmetry and current division.
- Rudy Severns, N6LF — Experimental Determination of Ground-System Performance for HF Verticals, Part 2: asymmetry and robustness of practical elevated systems.
- IEC 60364-5-54 — Earthing arrangements and protective conductors: the protective-earthing and bonding function.
- IEC 62305-1:2024 — Protection against lightning, General principles: the lightning-protection system boundary.
Practical Conclusion
Soil is real, variable and sometimes lossy; it is not simply the enemy. Radials and counterpoises give us deliberate control, but their count and length must serve the bands and geometry instead of fashion. A metal roof is a current-bearing structure, not a guaranteed mirror. An earth rod can join the RF network, but it is not an ideal RF sink. Protective earth remains a safety system.
When SWR, noise or pattern seems mysterious, do not ask for “more ground.” Draw every conductor, measure where the current went and improve the path that actually needs improvement.
Mini-FAQ
- Is soil always the enemy of an HF vertical? No. Soil is part of the electromagnetic environment and its loss varies with composition, moisture and frequency. Strong near-feed return current through lossy soil is undesirable, but the installed result must be measured.
- Is an earth rod a good RF return by itself? Not automatically. The rod, bonding conductor and soil form a distributed impedance, and the rod may simply add another RF-current branch. Keep required safety bonds and measure the antenna current path.
- How many on-ground radials should I install? There is no installation-independent count. Radiator geometry, bands, length, spacing, soil, total wire and diminishing returns all matter. Start with a defensible layout and measure before reallocating copper.
- Do a few elevated radials always outperform an on-ground field? No. Carefully balanced elevated systems can perform very well, but height, tuning, symmetry, nearby conductors and accessibility matter. N6LF’s comparisons belong to their tested geometries.
- Can a metal roof replace radial wires? It can carry useful current, but seams, dimensions, connection, height, cables and surrounding metal determine whether it behaves as a ground plane, parasitic element or unwanted coupling path.
- Where should a common-mode choke go? At a measured boundary where sufficient common-mode impedance will reduce the unwanted exterior-current path. No fixed distance works for every antenna, and the choke must not replace required bonding or lightning protection.