Six Feet of Radials Is Not a Universal Ground-System Rule
Six Feet of Radials Is Not a Universal Ground-System Rule
Rudy Severns, N6LF, measured real and modelled vertical-antenna ground systems. His work is valuable precisely because every result belongs to a particular radiator, frequency, soil, radial layout and measurement.
“Six feet of radials will fix everything” is the kind of sentence that survives because it is easy to repeat. It is not what N6LF established. Six feet has no engineering meaning until we specify the band, radiator, radial count, soil, conductor height, return-current boundary and quantity being compared.
My position: do not turn Rudy’s careful experiments into another magic number. Use his results to identify the mechanisms, then measure the antenna you actually installed. The best use of copper is installation-dependent, and neither “many short” nor “few long” is a universal answer.
The Conditions Belong to the Result
N6LF’s 2009 series was designed around controlled comparisons. Much of the work was performed at 7.2 MHz, with defined verticals and radial fields over measured soil. A distant receiving antenna measured relative transmission while the radiator was re-adjusted where necessary. Feedpoint impedance and radial-current division were also examined, and the feedline common-mode path was deliberately controlled.
That method can expose a small change between two configurations. It does not turn the result into an absolute efficiency, a complete radiation pattern or a rule for every band. Rudy explicitly described the work as a limited set of experiments rather than an answer to every possible ground system.
| Keep with the result | Why it matters |
|---|---|
| Frequency and radiator geometry | Electrical height, loading and base current determine how strongly the antenna excites the ground system. |
| Radial count, length, spacing and height | These variables change current distribution, soil coupling, feedpoint impedance and sometimes pattern. |
| Soil conductivity and complex permittivity | Surface radials are loaded by the soil, and the values move with composition, moisture, temperature and frequency. |
| Feedline and other return paths | Coax exterior, mast, bonds and nearby conductors can become unintended radials unless their currents are controlled and measured. |
| The measured quantity and reference plane | SWR, complex impedance, relative field, accepted power and efficiency are not interchangeable results. |
Why Sparse Long Surface Radials Can Misbehave
One of Rudy’s most useful results came from a sparse on-ground radial system, not from a universal preference for short wire. In Part 2, a 34-foot vertical was tested at 7.2 MHz with four insulated radials on the soil. Their equal length was varied from 33 feet down to 18 feet while impedance, relative transmission and radial current were observed.
The radials were not quarter-wave conductors in free space. Coupling to real soil changed their electrical behaviour. With only four, a loss-producing radial resonance appeared; shortening the wires moved that condition and improved the measured relative signal by several decibels in that installation. Similar behaviour appeared with eight radials.
The boundary matters. As the number of radials increased, sensitivity to that length change became much smaller. In the 32-radial comparison, the 33-foot set was about 0.12 dB ahead of the 21-foot set. The experiment therefore supports a narrow conclusion: a sparse surface screen can have a harmful length-dependent resonance. It does not show that short radials are inherently better.
The Equal-Wire Comparison Is Useful, but Conditional
Part 4 contains another result that often loses its context. At 7.2 MHz, Rudy compared sixteen 33-foot radials with thirty-two 17-foot radials under a top-loaded one-eighth-wave vertical. The two fields produced nearly the same relative signal even though the feedpoint impedances were substantially different. The radiator also had to be lengthened to restore resonance with the shorter field.
That experiment demonstrates a real design option. With a fixed amount of wire and limited space, distributing the conductor among more shorter radials can improve angular coverage near the base and may equal or outperform fewer longer radials for a particular antenna and soil. It does not establish an optimum count or length independent of the installation.
The same paper compared several verticals as 0, 4, 8, 16, 32 and 64 surface radials were added. The more severely shortened radiators benefited more from an aggressive ground system because their radiation resistance was lower and ground loss occupied a larger part of the accepted-power ledger. Most of the measured improvement in those tests arrived early in the sequence, with diminishing returns afterwards. Again, that is evidence from those antennas and that soil—not permission to label one count “enough.”
Six Feet Is a Dimension, Not a Design
A fixed 1.8 m radial can be a useful conductor in one system and a poor use of space in another. Its electrical length changes with frequency, insulation, height and soil coupling. Its value also depends on how many such conductors share the return current and how evenly they cover the area around the feedpoint.
The popular claim that most loss occurs inside one fixed fraction of a wavelength is too simple to design from. Current density is often greatest near the base of a ground-mounted vertical, so close-in conductor coverage matters. But the useful radius is not a hard 0.125-wavelength boundary. It moves with radiator current, radial geometry, soil and frequency. Extending a field can still reduce loss, while a sparse long field can encounter the resonance that Rudy measured.
There is also no defensible conversion from “six feet” to a band-by-band performance table without defining the rest of the system. A centimetre-perfect list can create false confidence while leaving out the variables that dominate the result.
A Conductive Screen Is Part of the Return System
Conductive mesh or cloth beneath a feedpoint should not be described merely as shielding. When it is electrically connected, it can carry and redistribute RF return current. Its effect depends on radius, mesh spacing, sheet resistance, bonds, contact with soil and how it overlaps the radial field.
In one Part 4 comparison, adding a conductive screen of roughly one-eighth-wavelength diameter to a top-loaded vertical that already had 64 radials produced no detectable improvement. Rudy noted that a screen might be more useful when the radial field is sparse. That finding does not prove that every mat is useless, and it certainly does not support a “ten times” performance claim. Compare the actual screen and radial layouts at the same accepted-power plane.
Elevated Radials Are a Different Coupled Antenna
An elevated radial system cannot be treated as a smaller version of a surface field. The elevated conductors participate strongly in the radiating structure. Their length, height, slope, angular symmetry and nearby objects affect current division, feedpoint impedance and pattern.
In N6LF’s controlled 7.2 MHz Part 3 experiment, four tuned radials elevated 48 inches were close in relative signal to a 64-radial surface system. That is a useful result for the tested antenna and site. It is not proof that four elevated radials at any height always equal a large ground field. Small mechanical and electrical asymmetries matter more when only a few elevated conductors carry the return current.
Elevated conductors can also develop substantial RF voltage and must be kept away from people, animals and unintended contact. Their RF function does not replace protective earthing, lightning bonding or the site’s electrical-safety design.
Ground Loss and Far-Ground Pattern Are Different Questions
Improving the radial field can reduce near-feedpoint loss and increase the field produced for the same accepted power. It cannot change the soil kilometres away from the antenna. The far-ground conductivity and permittivity, terrain and obstructions still affect low-angle reflection and the elevation pattern.
This is why a good near-ground system does not guarantee a particular take-off angle. ITU-R BS.705-2 treats soil, topography and surrounding structures as parts of the installed HF pattern. If the claim concerns direction or elevation angle rather than only input impedance, pattern evidence belongs in the test.
Input Resistance Is Not a Ground-Loss Meter
A common shortcut is to subtract an assumed radiation resistance from measured feedpoint resistance and call the remainder ground loss. That fails when the radial system changes the radiator current distribution, feedpoint reference current, common-mode path or radiation resistance itself.
Low SWR is equally inconclusive. A lossy system can be easy to match. A useful comparison records complex impedance at a declared plane, accounts for tuner and feedline loss, controls exterior-coax current and compares the same accepted power. Remote field measurements should include an A/B/A restoration so that soil moisture, geometry and propagation drift do not masquerade as improvement.
A Radial Test Worth Believing
- Define the objective. State whether the comparison concerns loss, realised field, pattern, bandwidth, impedance stability, land use or copper cost.
- Describe the radiator. Record height, loading, conductor dimensions, feedpoint, mast and nearby metal.
- Describe every radial. Record count, individual length, angle, elevation, insulation, bonding and total conductor.
- Characterise the ground. Use measured or defensible conductivity and permittivity ranges, and record moisture and weather.
- Control parallel paths. Characterise the feedpoint choke and map current on the coax exterior, mast and bonds.
- Keep the measurement plane fixed. Save R + jX and compare accepted power, not transmitter setting or SWR alone.
- Measure current distribution. Radial-current imbalance or an outward-moving current maximum can explain a result hidden at the feedpoint.
- Verify remotely. Use repeatable field or receive measurements with A/B/A restoration, and measure pattern when pattern is the claim.
- Repeat by band. A multiband installation is a different electrical system at every frequency.
Primary Evidence
- Rudy Severns, N6LF — Experimental Determination of Ground System Performance for HF Verticals, Part 1: test method, instrumentation, repeatability and measurement limits.
- Rudy Severns, N6LF — Experimental Determination of Ground System Performance for HF Verticals, Part 2: excessive loss and resonance in sparse surface-radial systems.
- Rudy Severns, N6LF — Ground System Performance for HF Verticals, Part 3: controlled elevated-versus-surface radial comparisons.
- Rudy Severns, N6LF — Ground System Performance for HF Verticals, Part 4: radiator dependence, radial count, equal-wire comparison and conductive screen.
- Rudy Severns, N6LF — Ground System Experiments: author’s summary of the experimental scope, near and far ground, and principal findings.
- ITU-R P.527-6 — Electrical Characteristics of the Surface of the Earth: frequency-dependent conductivity, permittivity, moisture, temperature and layered-ground guidance.
- ITU-R BS.705-2 — HF Transmitting and Receiving Antennas: antenna-pattern treatment including ground, terrain and surroundings.
Joeri’s Bottom Line
What Rudy gave us was better than a recipe. He showed why sparse surface radials can become resonant and lossy, why more conductors can reduce sensitivity to length, why equal wire can be distributed in different useful ways, and why elevated radials demand a different analysis.
So six feet does not fix everything. Nor does one fashionable count. Put the conductor where the installed current needs it, keep the soil and radiator attached to the decision, control the unintended return paths, and verify the result at the same measurement plane.
Mini-FAQ
- Did N6LF find that 1.8 m is the ideal radial length? No. He tested defined radial lengths, counts, radiators, frequencies and soils. A fixed 1.8 m length has no universal performance meaning.
- Did more short radials always beat fewer long ones? No. One equal-wire comparison produced nearly the same relative signal with more shorter radials, but impedance and radiator tuning changed. Other configurations produced different results.
- Why did shortening sparse surface radials sometimes help? Soil loading moved the radial system’s electrical behaviour. With only four or eight radials, a length-dependent resonance increased radial current and loss; changing length moved that condition.
- Can conductive mesh replace a radial field? Sometimes mesh can contribute useful RF return-current coverage, but its effect depends on size, conductivity, bonds, soil and the surrounding radial system. It must be compared as part of the installed antenna.
- Are four elevated radials always equivalent to 64 surface radials? No. A close result was measured in one controlled 7.2 MHz installation. Elevated radial height, tuning, symmetry, surroundings and feedline current remain part of the result.
- What should I measure before changing a radial system? Record geometry, soil condition, R + jX at one plane, radial and coax-exterior currents, accepted power and repeatable remote field. Add pattern measurements when direction or elevation is the claim.