The Paper That Made Baluns Stop Being Magic

Roy Lewallen’s Balun Paper, Re-Read by ON6URE

The Paper That Made Baluns Stop Being Magic

Roy Lewallen’s article Baluns: What They Do And How They Do It belongs next to Sevick, Maxwell, Guanella, Ruthroff, and the other classics.

Not because it gives the fanciest winding recipe.

Not because it sells a miracle core.

But because it starts with the correct question:

What problem is the balun supposed to solve?

That sounds obvious. It is not. Most balun discussions start with ratios: 1:1, 4:1, 9:1. Others start with labels: balun, unun, voltage balun, current balun, Ruthroff, Guanella. That label-first thinking is exactly why we wrote The Great BALUN / UNUN Confusion — Why the Labels Mislead.

Lewallen starts somewhere better: the antenna system itself.

That is why his paper still matters.

The Real Problem: The Feedline Became Part of the Antenna

Lewallen’s central point is simple: the bad symptoms are not caused by the word “unbalanced.” They are caused by unwanted current where it does not belong.

Pattern distortion. TVI. RF in the shack. A bridge reading that changes when you move your hand near the coax. These are not mysterious SWR effects. They are signs that the feedline is participating in the antenna system.

Inside a coaxial cable, the center conductor and the inside of the shield carry equal and opposite currents. That is the wanted transmission-line mode. The trouble begins when current also flows on the outside of the shield. At that moment, the coax is no longer only a feedline. It has become an extra antenna conductor.

This is also the core message in Baluns in a Nutshell: isolation matters as much as transformation. A transformer can change impedance and still leave the station dirty. A choke can stop unwanted feedline current without changing impedance at all. They are different jobs.

That is exactly the trap many hams still fall into today.

The box says “BALUN.”

The coax still radiates.

The shack still has RF.

The receiver still hears every LED lamp in the house.

So the label did not solve the problem.

The Missing Third Conductor Problem

There is another practical detail that is often missed in simple dipole explanations.

Even when the dipole itself is mechanically and electrically well balanced, feeding it directly with coax can disturb that balance. One dipole leg is connected to the coax center conductor. The other dipole leg is connected to the coax shield. The wanted transmission-line current flows on the inside of that shield, opposite the center conductor current.

But the outside of the shield is also physically connected to that same feedpoint terminal. Unless something stops it, the outside of the coax shield becomes an additional available conductor attached to one side of the dipole.

In practical terms, the feedpoint no longer sees only two antenna legs. It sees two intended dipole arms plus an unintended third path: the outside of the coax shield.

That third path does not need permission. If the geometry, impedance, coupling, or station layout allows current to flow there, it will. The result is feedline radiation, pattern distortion, RF in the shack, noise pickup on receive, or a measurement that changes when the coax is moved.

This does not mean every unchoked dipole instantly becomes unusable. It means the installation is no longer fully controlled. Sometimes the symptoms are obvious. Sometimes they are hidden. Sometimes the coax route happens to reduce the problem. But in a repeatable HF station design, relying on luck is not engineering.

A coax-fed dipole without a feedpoint choke is not just a two-leg antenna anymore; the outside of the coax shield is an extra available conductor attached to one side of the feedpoint.

For ON6URE, this makes the feedpoint choke on a coax-fed balanced antenna the default, not the exception.

Voltage Balance Is Not Current Balance

Lewallen makes a distinction that is still badly misunderstood: a voltage balun and a current balun are not doing the same job.

A voltage balun tries to force equal and opposite voltages at the balanced port. That sounds attractive, but antennas radiate from current, not from the label on a winding diagram.

If the two antenna halves are perfectly identical, equal voltage may produce equal current. But real antennas are rarely perfect. One side sees a tree. The other side sees the mast. One leg slopes differently. One side couples harder to the roof, gutter, tower, wall, rain gutter, or ground.

Equal voltage applied to unequal impedances gives unequal current.

And unequal current means imbalance.

This is the same practical distinction covered in Common Voltage vs. Current Balun Myths — Debunked. A voltage balun enforces a voltage relationship. A current balun suppresses the unwanted current path and pushes the antenna system toward equal and opposite currents. Those are not interchangeable jobs.

This is why many classic voltage baluns look reasonable on paper or into a symmetrical dummy load, but disappoint in a real installation. Lewallen’s experiments showed the same direction clearly: the voltage balun often improved the situation, but the current balun did the better job of controlling imbalance current.

The Current Balun Is the Station Tool

The current balun, or choke balun, attacks the real station problem.

It does not merely create pretty voltages. It resists the unwanted current path and helps stop the feedline from becoming part of the antenna. On a coax-fed dipole, that means choking the outside of the coax shield so it cannot behave as an unintended third conductor connected to one side of the feedpoint.

That is why the Guanella transformer remains so important in real HF systems. Used correctly, it behaves as a current-mode device. It can transform impedance, but its more important station-level role is current balance and common-mode suppression.

This is also where language creates confusion. A “balun” can be an impedance transformer, a choke, a voltage-forcing device, a current-forcing device, or a mixture of several functions. The label alone tells you almost nothing unless you know the topology and the job it performs in the complete antenna system.

Coax Is Not the Enemy

Another strong point in Lewallen’s paper is that coaxial cable is not “bad” simply because it is coax.

Coax can carry a clean differential-mode signal internally. The center conductor and the inside of the shield form the transmission line. The outside of the shield is a different story. If the antenna system gives that outside surface a reason to carry current, it will.

So the real question is not:

Is coax balanced or unbalanced?

The real question is:

Is there unwanted current on the outside of the shield?

That is a much better engineering question.

It also explains why “common mode” is such an abused phrase in ham radio. In strict EMC language, common-mode current has a specific meaning. In ham antenna work, we often use the phrase loosely to describe unwanted feedline current, outer-shield current, or imbalance current. That difference matters, and we explain it in Why “Common Mode” Is the Most Abused Term in Ham Radio.

Stop blaming the cable.

Look at the system.

Look at the return path. Look at the feedpoint. Look at antenna symmetry. Look at choke placement. Look at station grounding and routing.

The current tells the truth.

Balanced Line Is Not Immune Either

Lewallen also made a point that still surprises people: the type of feedline does not automatically decide whether the system is balanced.

Twin lead, ladder line, or open-wire line can also radiate when the conductor currents are not equal and opposite. Geometry helps, but geometry is not magic. A balanced-looking line can still become part of the antenna system when one side couples differently to ground, metalwork, trees, walls, or the tuner.

That is exactly the point in When Open-Wire Feedline Starts to Radiate: radiation is not a “coax-only” problem. It is a current-balance problem.

This is where many old rules become dangerous shortcuts. “Use ladder line and the problem disappears” is not engineering. It is folklore. The feedline behaves when the currents behave.

Where ON6URE Goes Further

Lewallen gave the principle. Modern station work needs the deployment rules.

That is where the ON6URE view extends the older paper.

A current choke at the feedpoint is often correct for a center-fed dipole. In fact, with a coax-fed dipole, it should be the default because the outside of the coax shield otherwise becomes an uncontrolled additional conductor attached to one side of the antenna.

But an end-fed half-wave may need the coax shield to act as part of the return path for a short distance. In that case, choking directly at the transformer can change the antenna behavior, move the problem elsewhere, or force the return current into a less predictable path. The better point may be farther down the coax.

This is why choke placement is not a slogan. It is system-dependent. Feedpoint, shack entry, tuner output, vertical base, EFHW transformer, open-wire transition — each location answers a different current-path problem. See Optimal Placement of Common-Mode Chokes for Various Antenna Types for the practical version.

A tuner output is another case. A voltage transformer or impedance transformer may be useful, but it does not automatically solve common-mode current. In high-impedance or mismatched environments, the choke must also survive real voltage, real current, real duty cycle, and real ferrite heating. That is why tuner baluns need different thinking than a simple feedpoint choke. See Correct Use of RF.Guru Antenna Tuner Baluns.

That is the modern correction:

Do not ask only whether it is a balun or an unun. Ask what job it actually performs in the antenna system.

Ruthroff Is Not “Bad” — It Is Just a Different Tool

One common mistake is to read Lewallen as “voltage balun bad, current balun good.” That is too simple.

A Ruthroff-style voltage transformer can be an excellent impedance transformer. It is often compact, efficient, and useful in EFHWs, Windoms, random wires, and other systems where voltage transformation is the main job.

But it is not automatically an isolation device.

That distinction is the whole point of The Ruthroff Transformer Explained. A Ruthroff transformer can solve an impedance problem while leaving a current-path problem unsolved. So when RF cleanliness matters, it must be paired with a proper choke or used inside a topology that handles unwanted current deliberately.

So no, Ruthroff is not the enemy.

Misusing Ruthroff as a choke is the enemy.

Why Lewallen Still Matters

Lewallen’s paper is valuable because it removes the magic.

A balun is not automatically good.

A voltage balun is not automatically wrong.

A current balun is not automatically perfect.

The correct question is always the same:

What unwanted current path exists, and what does this device do about it?

That is the part many articles skip. Lewallen did not skip it. He measured it.

His conclusion is still useful today: the current balun gave superior balance in his experiments, and the voltage balun, while sometimes helpful, did not solve the real antenna-system problem as reliably.

What Lewallen Got Spot-On

• Feedline radiation is caused by unwanted current, not by the word “unbalanced.”
• Coax outer-shield current is a separate problem from the wanted current inside the coax.
• A coax-fed dipole can become a three-conductor system when the outside of the shield is left unchoked.
• Balanced-looking feedline can still radiate if conductor currents are not equal and opposite.
• Voltage balance does not guarantee current balance.
• A current balun is the right tool when the real problem is feedline current.
• Measuring current tells you more than trusting the label on the box.

What Needs an ON6URE Update

• Use “common mode” carefully. In many ham antenna cases, we are really discussing outer-shield current, imbalance current, or stray return current.
• For a coax-fed balanced antenna, treat the feedpoint choke as the default. Without it, the outside of the coax shield is an uncontrolled extra conductor attached to one side of the feedpoint.
• Do not treat choke placement as universal. Feedpoint, shack entry, EFHW return path, vertical radials, and tuner output are different cases.
• Do not assume a transformer also provides isolation. Transformation and choking are separate functions.
• Do not judge a balun only by SWR or a dummy-load test. The real antenna system may be reactive, asymmetric, and frequency-dependent.
• Do not let the feedline become an uncontrolled part of the antenna. In practical HF installations, a deliberate choke strategy is part of making the antenna system repeatable.
• Power handling is not just “watts.” It depends on voltage, current, duty cycle, mismatch, ferrite loss, and unwanted current paths.

Takeaway: Lewallen Through the ON6URE Lens

Lewallen solved the balun myth at the current level.

ON6URE extends it to the station level.

• Lewallen: identify the imbalance current and choose the balun that actually reduces it.
• ON6URE: then place the choke or transformer where the station current path actually needs it.

The old lesson still stands:

A balun that does not control unwanted current is not solving the main station problem. It may transform impedance. It may create equal voltages. It may look good in a schematic.

But if the coax shield still radiates, the station is not fixed.

For ON6URE, this is why choking is not an optional afterthought in HF antenna work. It is part of defining the antenna boundary. It helps keep the radiation pattern intact, keeps return currents where they were intended to be, reduces RF in the shack, and makes the installation more predictable from one environment to the next.

Words matter. Ratios matter. But current matters more.

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