Why the PA0FRI “Coax Balun 1:4 and 1:1” Document Keeps Coming Back

Article update: Technical review and correction of a widely circulated 2019 balun document.

— and Why It’s Technically Flawed

I regularly get this document forwarded to me by email. Almost every time, it triggers a back-and-forth discussion — not because the construction ideas are unusual, but because the author, PA0FRI, was a prominent and respected figure in the Dutch amateur radio community.

That prominence is exactly why this document keeps resurfacing. Authority bias is powerful, and once a text gains “classic” status, its technical claims often stop being questioned.

The document discussed here is:
https://www.pa0fri.com/Ant/Balun/balun.htm ( pdf version )

Before going further, one important clarification for ease of reading:

Throughout this article, the term common-mode is used as a practical umbrella term for the unwanted currents discussed in the document.

In strict textbook terms, there is a difference:

• Common-mode currents in the electromagnetic sense have no defined return path and are referenced to the surrounding environment.
• Stray return currents in transmitting systems often do have a defined reference and arise from imbalance rather than pure common-mode excitation.

However, from the perspective of measurement, system behavior, radiation effects, and mitigation, both manifest in the same way: current flowing where it should not, on conductors that were never meant to radiate. That is why, in the context of this document and its tests, treating them together as “common-mode problems” is appropriate — even if a textbook would separate the definitions.

It is worth noting that the principle of enforcing balance through current symmetry is not new. Long before many of the constructions discussed in this document circulated, ON7FU already emphasized that true balance can only be achieved by forcing equal and opposite currents — not by relying on voltage division or apparent symmetry.

What This Device Is — and What It Is Not

The construction itself — two equal lengths of coax, parallel on one side and series-connected on the other — is a valid transmission-line transformer topology. The impedance ratios quoted in the document are mathematically correct.

The problem starts when differential behavior and common-mode behavior are implicitly treated as the same thing. They are not.

A balun used between an open-wire line and a tuner is primarily judged by its ability to suppress common-mode current. Equal currents in each conductor do not prove this suppression.

The Fundamental Measurement Error

The document compares baluns by measuring current in each conductor separately and concludes that the system is “well balanced.”

This cannot reveal common-mode behavior.

Pure common-mode current produces equal, in-phase currents on both conductors. Two meters showing the same reading can just as easily indicate a severe common-mode problem.

Common-mode must be measured by:

• Placing a current probe around both conductors together (open-wire line), or
• Measuring current on the outside of the coax shield.

In addition, common-mode currents form standing waves. Measuring at a single point along the line is insufficient and can be actively misleading.

“No Core, So No Power Problems” Is Incomplete

The document suggests that the absence of ferrite eliminates power-handling concerns.

While it is true that ferrite saturation is avoided, this ignores an important reality:

A coaxial cable carrying only differential current is a well-contained TEM structure. Once common-mode currents exist, that containment no longer holds.

In air-wound coax chokes, common-mode excitation creates displacement currents. These displacement currents couple to the surrounding space, effectively making the outer surface of the coiled coax electrically active.

At that point, the coil is no longer just “coax” — it becomes an exposed RF structure with:

• Radiation,
• Parasitic capacitance,
• Self-resonant behavior.

This is why air-wound coax baluns often behave unpredictably across frequency.

Why Dismissing Network Analysis Is a Red Flag

The document dismisses laboratory measurements and network analyzers as something “experts misuse.”

In reality, the issue is not the instrument — it is the measurement model.

At RF.Guru we use a Y-parameter based approach for common-mode choke characterization, specifically the Y21 method:
Why the Y21 method is the only ham measurement that actually works

This method directly measures common-mode coupling and impedance across frequency, revealing resonance, loss, and usable bandwidth in a way that SWR or single-port tests never can.

Air-Wound Coax Chokes Are Resonant by Nature

A coiled coaxial cable is an inductor with parasitic capacitance. That combination inevitably produces a self-resonant frequency.

Below resonance, impedance may rise. Near resonance, behavior becomes erratic. Above resonance, the structure can become capacitively dominant and stop behaving as a choke altogether.

Broadband claims without common-mode impedance measurements are therefore meaningless.

Reactance Is Not Loss

One of the most serious errors in the document is the claim that an inductive reactance comparable to the line impedance will dissipate a large fraction of transmitter power as heat.

Reactance does not dissipate power. Only resistance does.

Loss depends on ESR and Q — neither of which are analyzed or measured in the document. Confusing reactance with loss invalidates the entire argument built on it.

The 4:1 Balun and the Tuner Myth

A 4:1 balun placed before a tuner is not a universal efficiency improvement. It simply reshapes impedance.

Sometimes that helps. Often it increases current, loss, and sensitivity to resonance. Notably, the document itself acknowledges that a 1:1 balun can be superior when low impedances are present.

Feedline Impedance vs. Feedline Loss — What Actually Matters

In a multiband system where you must use a tuner, the feedline’s characteristic impedance (300 Ω, 450 Ω, 600 Ω, etc.) is usually not the deciding factor for “can I tune it?”—the tuner will see whatever impedance the line transforms the antenna to, and the real question becomes how much loss you pay while doing that. That’s why open-wire and open-line feedlines are so powerful: they are very low loss even with high standing waves, so “mismatch on the line” is not automatically “power turned into heat.” Coax, by contrast, has meaningfully higher attenuation and its loss rises sharply when it is carrying large standing waves. Any significant length of coax on the high-SWR side is exactly where efficiency gets burned.

Using a coax-made “balun” as the interface to feed open-wire line is therefore a poor engineering default—not because TEM transmission itself is inefficient (both coax and two-wire lines are TEM or quasi-TEM at HF), but because a coax-wound transformer or choke brings along coax’s dielectric and conductor loss, plus a shield that is capacitively coupled to the surroundings and can easily support common-mode current unless the design provides high common-mode impedance over the whole bandset.

Changing Coax Length Is Not a Solution

Adjusting coax length to make a system “tune” merely uses transmission-line transformation to move impedance around.

It does not solve imbalance or common-mode problems — it only relocates them.

Conclusion

This document persists because of its author’s reputation, not because its technical reasoning is sound.

The core issues are consistent throughout:

• Common-mode behavior is never properly measured.
• Reactance is repeatedly confused with loss.
• Resonance and displacement currents are ignored.
• Proper measurement techniques are dismissed.

True forced symmetry at the transition between an unbalanced system (coax, ATU) and a balanced load (open-wire line or symmetric antenna) can only be achieved with a current-enforcing balun. This principle was already clearly articulated by ON7FU, my predecessor, and it remains the foundation of our own balun designs today. These devices do not rely on apparent balance or voltage division — they actively force equal and opposite currents by presenting a high, measurable common-mode impedance while preserving a defined differential transmission path.

Respect for contributors to the hobby does not require us to preserve incorrect RF theory.

For deeper background, see all our:
Technical deep dives
Baluns and UNUNs — technical articles
Ham radio myth debunking
Transmission line fundamentals

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