When a Better Choke Makes the SWR Look Worse

Every now and then, a real-world measurement tells a much more useful story than a clean laboratory graph. This case, shared by Jaap / PA0LJD, is a good example.

The installation was an 80/20 off-center-fed dipole using a 1:4 UNUN and a common-mode filter close to the feedpoint. The problem appeared mainly on 10 meters. In Jaap’s installation, his own symmetrical bifilar construction produced a lower SWR than the RF.Guru Quad Core common-mode choke. At first sight, that looks like a simple conclusion: the bifilar choke seems “better.”

But RF rarely rewards simple conclusions. A lower SWR does not automatically mean better common-mode suppression. It does not automatically mean better antenna efficiency. And it certainly does not prove that less power is being lost.

The Measurement Setup Was Not Just Measuring a Choke

In our own test setups, we usually observed the opposite effect: the newer short-pigtail common-mode choke behaved more predictably and gave a more stable result than the older longer-pigtail version. That contrast is exactly why this case is interesting.

At HF, especially on the higher bands, it is almost impossible to perfectly reproduce an RF installation. We are not just measuring an antenna and a choke. We are measuring a complete electromagnetic system.

The antenna wire, the UNUN, the pigtail, the coax shield, the choke, the roof, the mast, nearby metal, the cable route, the shack grounding, the moisture in the structure, and everything inside the near field can influence the final result. In an off-center-fed antenna, that influence can become very visible on 10 meters.

The Three Original Configurations

Three relevant configurations were compared:

• the newer RF.Guru short-pigtail common-mode filter with integrated ferrite treatment;
• the older RF.Guru longer-pigtail common-mode filter with customer-added clip-on ferrites;
• a customer-built symmetrical bifilar construction, used between the UNUN and coax feedline.

Why the 10 Meter Result Changed So Much

On the lower HF bands, small lengths of coax or pigtail often appear less dramatic. On 10 meters, that changes. A pigtail or coax section of 30 to 50 cm is no longer electrically invisible. It can behave as a small reactive stub, an unintended counterpoise section, or a coupling element between the UNUN, the choke, and the surrounding structure.

That is especially true in an off-center-fed dipole. The feedpoint is not at the current symmetry point, the two arms are not equal, and the antenna impedance varies strongly from band to band. On 10 meters, small geometry changes can shift the measured impedance enough to decide whether the internal tuner accepts the load or not.

In Jaap’s data, the older long-pigtail arrangement and the newer short-pigtail arrangement do not merely differ by “better or worse choking.” The electrical reference point of the choke has changed. That changes what part of the coax shield, pigtail, and near-field coupling remains active.

Why the Bifilar Construction Can Look Better

The most important trap in this case is the symmetrical bifilar construction. It may produce a nicer SWR curve, but that does not mean it is acting as a clean coaxial common-mode choke.

A proper coaxial common-mode choke should leave the wanted differential transmission-line mode largely undisturbed. In coax, the wanted RF current flows on the center conductor and returns on the inside of the shield. The unwanted current is the current that flows on the outside of the shield, or on other unintended return paths.

A coaxial common-mode choke primarily adds impedance to that unwanted outside-shield path. The internal coaxial transmission mode should remain a 50 Ω transmission path.

A bifilar pair is different. Two parallel wires wound together form a coupled transmission-line structure with interwire capacitance, leakage inductance, ferrite coupling, self-resonance, and a characteristic impedance that is not the same as a normal 50 Ω coaxial line. On the higher HF bands, that structure can easily become more than a choke. It can become a lumped reactive component in the antenna system.

The Bifilar Choke May Be Eating Power, Not Saving It

A symmetrical bifilar construction places common-mode impedance on both conductors of the pair. In theory, that may sound attractive. In a real coax-fed off-center-fed antenna, it can become problematic.

The antenna is already asymmetrical. The two wire sections do not carry identical current distributions. The feedpoint impedance changes from band to band. The coax shield and the environment may already be participating as a third path. In that situation, the bifilar construction may not only block unwanted current. It may also load part of the active RF system.

The resistive part of that ferrite impedance becomes heat. The reactive part shifts resonance or matching. The result can be a lower SWR at the shack while some of the transmitter power is being dissipated in ferrite, wire resistance, shield current paths, or nearby lossy objects instead of being radiated by the antenna.

That is why a transmatch in the shack may be a more predictable and possibly less lossy way to handle residual mismatch than allowing a bifilar choke to act as an unintended lumped matching element. A tuner is at least designed to transform impedance. A bifilar choke that accidentally behaves as a reactive matching component is much harder to interpret.

SWR Is Not Antenna Efficiency

This case also touches one of the most persistent myths in amateur radio: the idea that a better SWR means a better antenna.

SWR only describes how well the impedance presented to the transmitter matches the transmission line or tuner. It says nothing by itself about how much power is radiated. A dummy load can show a perfect SWR and radiate almost nothing useful. A lossy transformer, lossy loading coil, lossy ferrite arrangement, or lossy earth return can also make the impedance look friendlier while reducing radiation efficiency.

So when the bifilar construction produces a better SWR, we should not immediately conclude that it is the better RF solution. It may simply be changing the impedance in a way the tuner likes. That is not the same as reducing common-mode current, improving pattern stability, or increasing radiated power.

Why Clip-On Ferrites Can Still Change the Result

If this is not a simple “more common-mode current equals worse SWR” problem, why do clip-on ferrites change the measurement?

Because clip-on ferrites can alter the impedance of an unintended RF path. On 10 meters, even the short section between the UNUN and the choke can be active. Adding ferrites to that section may not fix the original antenna geometry. Instead, it may change the current distribution on a third path and retune the complete installation.

That is not useless. It can be a practical solution. But it should be recognized for what it is: installation-specific tuning of an unwanted current path, not proof that the antenna has become inherently more efficient.

What the Short-Pigtail Choke Actually Does

A shorter pigtail moves the common-mode choking point electrically closer to the UNUN. In a clean installation, this is generally desirable. It reduces the amount of coax shield that remains active between the transformer and the choke.

But if the previous longer pigtail was unintentionally helping the 10 meter match, removing that active length can make the SWR look worse. That does not automatically mean the new choke is poorer. It may simply mean the previous setup contained a hidden matching element.

In other words: the new choke can be electrically cleaner while the shack SWR becomes less attractive.

The Proposed Follow-Up Test

A useful next test would be to temporarily add 30 to 50 cm of coax between the UNUN and the newer short-pigtail common-mode filter. If the 10 meter SWR shifts back toward the older result, that would strongly suggest that the earlier configuration was being helped by extra electrical length or reactive coupling.

Jaap performed that exact kind of follow-up test, and the result turned out to be one of the most useful parts of the entire case study.

The Follow-Up Test: Adding 50 cm of Coax

After the first comparison, Jaap / PA0LJD performed a very useful follow-up test. The goal was simple: if the pigtail length and the coax section between the UNUN and the common-mode choke are really part of the RF behavior on 10 meters, then adding extra coax should make the effect visible.

Two extra measurements were made:

• the newer short-pigtail common-mode filter with an additional 50 cm coax section between the CMC and the UNUN;
• the same setup, but with 10 clamp-on ferrites added over that 50 cm coax section.

This is an important result. The extra 50 cm coax without ferrites gives a clear degradation, which is exactly what we would expect if that coax section is no longer just a passive connection. On 10 meters, that short section can behave as an active RF element: a small stub, an unintended return path, or a coupling element between the UNUN, the choke, and the surrounding near-field environment.

When the same 50 cm coax section is fitted with 10 clamp-on ferrites, the situation improves again. That does not mean the coax has become a better antenna. It means the ferrites increase the impedance for unwanted current on the outside of the coax shield. Mechanically, the coax section is still present. Electrically, it is less free to participate as part of the antenna system.

This confirms the main lesson from the earlier measurements. The pigtail, the coax shield, the choke position, and the local environment form one RF system. Especially on 10 meters, they cannot be evaluated as separate parts.

It also explains why the symmetrical bifilar solution originally looked attractive on the analyzer. It was probably doing something similar, but in a less controlled way. It did not merely suppress common-mode current. It also shifted the impedance of the total system. The clamp-on ferrites on the added coax section show the same principle more clearly: when the unwanted path is given more impedance, the measured antenna behavior changes.

This does not mean SWR is now the final judge. The improvement with 10 clamp-ons is diagnostically useful, but it still does not tell us how much power is actually radiated and how much is dissipated in ferrite, shield current paths, or nearby lossy objects. For that, an RF current measurement on the outside of the coax shield would be the next useful step.

Why This Matters Beyond One Installation

This is not only about one OCFD installation. It is a wider lesson about how multiband antennas behave in the real world.

Near-field coupling is difficult to model. Roofs, masts, gutters, shack wiring, PE grounding, coax routing, and nearby structures can all become part of the RF system. That is why the same antenna and choke combination can behave differently in two locations.

A test setup in a cleaner environment may show one result. A sloped roof installation with nearby structures may show another. Neither result is automatically “wrong.” They are simply different RF systems.

Practical Lessons from This Case

• A lower SWR from a choke does not prove better common-mode suppression.
• A bifilar construction can behave as an unintended matching element on higher HF bands.
• The pigtail between UNUN and choke can become electrically active, especially on 10 meters.
• Adding 50 cm of coax between UNUN and choke can significantly change the 10 meter result.
• Adding clamp-on ferrites to that same coax section can improve the result by raising the impedance of an unwanted shield-current path.
• Clip-on ferrites may improve the measurement by altering an unintended current path, not necessarily by improving antenna efficiency.
• A proper current measurement on the outside of the coax shield is needed to evaluate common-mode suppression.
• SWR should never be used as a direct proxy for antenna efficiency.

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