Multiband Verticals and the 4:1 UNUN: Measure the Whole System

The argument is often reduced to folklore: one camp insists on a “magic” radiator length, while another says that almost any practical wire plus a 4:1 UNUN, radials and a tuner will perform just as well. Both shortcuts discard the installed impedance, loss, current distribution and pattern that decide the result.

The Rybakov Name Describes a Family, Not a Magic Dimension

A Rybakov-style antenna is commonly understood as a non-resonant or deliberately non-quarter-wave vertical conductor used across several bands with a return structure, an impedance transformer and a tuner. Implementations differ in radiator length, counterpoise or radial field, transformer, feedline and tuner location. The name does not define one electromagnetic system.

That makes the familiar 7.2 m, 7.6 m, 6 m or 9 m figures starting geometries, not performance certificates. The same physical wire represents a different electrical length on every band. It can be short on a lower band, approach a quarter or half wavelength elsewhere, and support several current maxima on an upper band.

I object to the word “magic,” but I do not replace it with “length does not matter.” Length changes feedpoint resistance and reactance, current maxima, voltage maxima, tuner load, transformer stress and elevation pattern. A useful multiband length is the one that avoids destructive load and pattern regions for the actual band set and installation—not the one repeated most often.

The Vertical and Its Return Structure Form One Antenna

A vertical is not merely a wire over a “lossy capacitor.” Current leaving the radiator terminal must return to the source through conductors and displacement paths. Depending on the installation, that network can include on-ground radials, elevated radials, soil, a deliberate counterpoise, the coax exterior, the mast, bonding conductors and nearby structures.

Those paths affect the terminal impedance because they are part of the antenna. They also decide how much accepted power becomes useful radiation, conductor or ground loss, or unintended feedline radiation. Moving the transformer 30 cm, rerouting coax or changing a sparse radial field can alter the result, but no fixed feedpoint height guarantees an improvement.

For on-ground systems, a free-space quarter wavelength is not a universal radial prescription. Soil loads the wires, and count, length, angular coverage, soil properties and radiator geometry interact. For elevated systems, a few radials are active antenna conductors whose length, symmetry, height, coupling and current balance require deliberate adjustment. Rudy Severns, N6LF, demonstrates both boundaries in his measured ground-system work.

What a 4:1 UNUN Actually Does

An ideal transformer with a 4:1 impedance ratio can transform an output load Zload towards an input value of approximately:

That relation transforms the complete complex impedance, not just resistance. A load of 120 + j160 Ω does not become a clean 30 Ω resistor; ideally it becomes 30 + j40 Ω before real transformer loss and parasitics are included. A tuner may still need to cancel reactance and complete the transformation.

Practical radiator lengths do not consistently land between 100 and 200 Ω across several bands. A multiband vertical can present values far below, within or far above that range as its electrical length and return network change. A 4:1 ratio can improve the tuner’s load region on some bands and make it worse on others.

Transformation and Common-Mode Control Are Separate

In the deliberately unbalanced installation described here, my default is to keep the two functions visible: select an UNUN from the measured differential load, then use a separately specified 1:1 choke to define where current on the feedline exterior should become small.

This is not a declaration that every vertical requires 4:1 transformation. It is not a claim that an UNUN plus choke automatically balances a badly defined antenna. A suitable current balun remains valid for a demonstrably balanced installed load, and an integrated transformer-and-choke assembly can work when transformation, balance, common-mode impedance and powered stress are qualified together.

The choke does not belong at a universal 0.05 wavelength from the transformer. If a declared section of coax exterior is an intentional return branch, the first effective choke can define the far end of that branch. If radials complete the intended antenna at the feedpoint, a feedpoint choke is a reasonable first candidate. In both cases, measure exterior current at several positions and repeat the map on every band.

A Good Match Is Not the Same as an Efficient Antenna

A tuner can present an acceptable impedance to the transmitter while power is lost in the transformer, tuner, feedline, conductors or ground. Feedline attenuation can also improve the SWR observed at the shack by absorbing forward and reflected power. The number on the radio therefore does not separate radiation from loss.

Compare power at declared reference planes:

• Transmitter output: the power entering the station-side network.
• After the tuner and feedline: the power that reaches the matching assembly under the actual standing-wave condition.
• Accepted at the antenna port: forward power minus reflected power at that named plane.
• Radiated: accepted power minus transformer, conductor, return-network and coupled losses.

A low SWR answers only the mismatch question at its measurement plane. It cannot by itself prove efficiency, gain, low ground loss or a clean pattern.

Multiband Means a New Pattern on Every Band

As a vertical conductor becomes electrically longer, its current distribution develops additional maxima and minima. The elevation pattern can split into lobes, and useful energy can move away from the low elevation angles wanted for a particular path. Nearby conductors, bends, the radial field and common-mode current can also disturb azimuth symmetry.

This is why “it tunes from 40 through 10 metres” is not the same claim as “it radiates efficiently with the desired take-off angle on every band.” Tuning establishes a load condition. Pattern and realized gain require a complete model with defensible ground data or a controlled measurement at equal accepted power.

A practical length may be chosen because its load set is easier on the tuner and transformer, because it avoids a pattern null in a required direction, or because it fits the support. Those are legitimate objectives. They should be named rather than compressed into “works better.”

Radials Must Be Designed for the Installed Geometry

“Four resonant radials per band” is not a universal requirement for a ground-mounted multiband vertical. It mixes the logic of a sparse elevated system with conductors lying on real soil. N6LF’s experiments show that sparse surface fields can develop length-dependent loss, that increasing count can reduce sensitivity to individual length, and that equal total wire can be distributed in different ways with different results.

The opposite slogan—many short radials always beat fewer long ones—is not universal either. Keep these variables attached to every result:

• frequency and radiator current distribution;
• radial count, individual length and angular spacing;
• on-ground, buried or elevated placement;
• soil conductivity and complex permittivity;
• feedline, mast and bonding paths; and
• the measured quantity: impedance, current, field, efficiency or pattern.

Choose the radial system from the available footprint and the complete band objective. Then verify it. Copper length is useful only when its installed current reduces loss or shapes the field in the way the station needs.

Commission the Complete Vertical, Not the Parts List

1. Define the objective. Name the bands, paths, tuner, power, duty cycle, site footprint and acceptable pattern compromises.
2. Record the complete geometry. Include radiator length and route, feedpoint height, radial field, mast, coax, bonds and nearby conductors.
3. Measure the untransformed load. Save calibrated R + jX at the antenna-side reference plane on every intended band.
4. Test candidate ratios. Compare bypass and candidate transformers with representative complex loads, not only a 50 Ω fixture.
5. Check the tuner’s real load domain. Confirm that the transformed impedances lie within the manufacturer’s usable region at the required power and frequency.
6. Declare the intended return. State whether radials or a counterpoise complete the feedpoint, or whether a controlled coax-exterior section belongs to the antenna.
7. Map common-mode current. Scan several marked feedline positions before and after the proposed choke boundary on every band.
8. Measure loss and stress. Record accepted power, transformer and tuner loss, voltage, current and equilibrium temperature at the intended waveform and duty cycle.
9. Verify the pattern claim. Use a complete model or controlled equal-accepted-power field measurements; do not infer elevation angle from SWR.
10. Restore the baseline. Use A/B/A trials so soil moisture, cable movement, temperature and propagation do not become the apparent improvement.

Primary Engineering Sources

• Roy Lewallen, W7EL — Baluns: What They Do and How They Do It: port balance, feedline imbalance and current-balun behaviour.
• Rudy Severns, N6LF — Radial System Design and Efficiency in HF Verticals: NEC study of ground-system count, length, soil and wire-budget tradeoffs.
• Rudy Severns, N6LF — Experimental Determination of Ground System Performance for HF Verticals, Part 2: measured sparse surface-radial resonance, length and count effects.
• Rudy Severns, N6LF — A Closer Look at Vertical Antennas With Elevated Ground Systems: elevated-radial coupling, symmetry and robustness.
• ITU-R P.527-6 — Electrical Characteristics of the Surface of the Earth: frequency-, moisture-, temperature- and layer-dependent Earth properties.
• Keysight — Impedance Measurement Handbook: complex impedance, calibration, fixtures and frequency-dependent equivalent circuits.
• Tom Rauch, W8JI — Common Mode Current: installed balance, feedline exterior current and current-path diagnosis.

Joeri’s Bottom Line

I will not defend a sacred 7.2 m wire, and I will not promise that 6 m or 9 m “works perfectly” because a tuner found a match. A multiband vertical is a coupled radiator, return system, transformer, choke, feedline and tuner. Each band sees a different version of that system.

A 4:1 UNUN is useful when the measured loads say it is useful. Radials are useful when their current and field contribution say they are useful. A choke is useful where it ends the intended return path. Measure those functions separately, then judge the complete antenna by loss and pattern at equal accepted power. That is the difference between a convenient multiband vertical and a lucky SWR trace.

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