Coax Length Before the Choke: Why It Matters for EFHW Antennas

Why EFHW and End-Fed Off-Center Systems Do Not Ask the Same Thing from a Common-Mode Choke

Reference: Werner Schnorrenberg – DC4KU, “Endgespeister Dipol mit Gegengewicht und Mantelwellensperre” (2019). Field-verified by RF.Guru designs and measurements.

A lot of confusion around end-fed antennas starts when people treat every choke as if it were doing the same job. It is not. In one installation, the choke is supposed to keep the feedline out of the antenna. In another, it is deliberately placed farther away so a short section of coax or a short counterpoise wire can serve as the intended return path.

Those are not the same electrical situation, so they do not want the same placement strategy.

The clean way to think about it is this: common-mode control is really return-path control. Once you decide where you want the return current to flow, choke placement becomes much less mysterious.

The First Mistake: Confusing Feedline Isolation with Return-Path Design

An EFHW is fed at a very high impedance point. That means the current at the feedpoint is relatively small, but it does not mean the need for a return path has disappeared. If you do not provide a deliberate return conductor, the outside of the coax shield usually becomes part of that return path.

That is why so many end-fed antennas appear to “work fine” until the feedline routing changes, the shack wiring changes, the weather changes, or a different band is selected. The missing half of the circuit was never eliminated ... it was merely uncontrolled.

Why a Ground Stake Is Not a Magic Cure

One of the oldest myths is that you can simply connect the transformer box to ground and call the problem solved. DC4KU’s measurements show why that is too simplistic. A ground lead can become just another conductor involved in the RF current distribution. It does not automatically force the coax to stay quiet.

Electrically, this makes sense. RF current does not read product labels. It divides among available paths according to impedance. If you add an earth lead without actually controlling where the current maximum sits and without isolating the feedline properly, you have not created order ... you have just added another path.

Why 0.05λ Keeps Showing Up

The familiar 0.05λ rule did not appear from nowhere. Steve Yates, AA5TB, described how a short counterpoise of at least about 0.05 wavelength can be enough for an end-fed half-wave. DC4KU then tested practical variants of that idea.

The key detail people often miss is that a short counterpoise by itself is not the whole story when you are using the common 49:1-style autotransformer arrangement. If the transformer and coax are still galvanically tied in the usual way, some return current can still prefer to flow on the outside of the coax unless you raise the impedance of that path with a proper choke.

That distinction is exactly why choke placement right at the box and choke placement after a short 0.05λ return section can produce very different results. They are solving different versions of the same problem.

Recommended 0.05λ Return Lengths Before the Choke

Indicative values below use a coax velocity factor of 0.86. For a separate wire counterpoise, use the physical wire length directly rather than the coax velocity factor. Real installations vary with height, soil, routing, nearby structures, and band.

These values are not magic lengths. They are practical starting points for defining the intentional return section before the choke. The exact optimum can shift with transformer design, feedpoint height, wire layout, band, coax type, and nearby structures.

Can You Roll Up Excess Coax?

Yes, but do it with intention. If the coax section before the choke is meant to serve as the intentional return section, its length and placement matter. After the choke, the coax should ideally be “cold” with respect to common-mode current, so excess coax behind the choke can usually be routed or loosely coiled with far fewer consequences.

Best practice:

• Do not tightly coil the intentional return section directly near metal objects.
• Keep the defined section before the choke physically predictable.
• Place the choke where you want the feedline participation to stop.
• Use a proper counterpoise, radial, or stainless ground rod when the design requires it.
• Do not assume coiling coax is automatically a choke; a random coax coil is not the same as a measured ferrite common-mode choke.

DC4KU’s Measured Configurations at a Glance

(Indicative) — values below are simplified from DC4KU’s 7.1 MHz / 10 W example to illustrate current distribution and relative behavior. Real installations vary with height, feedline routing, surroundings, transformer design, and band.

What This Means for EFHW, EFOC, and Off-Center-Fed Systems

An EFHW with no deliberate counterpoise is the most misunderstood case. In that form, the outside of the coax is usually doing real RF work. So a choke placed blindly right at the transformer can be counterproductive if it blocks the only return path you actually had.

An EFHW with a deliberate short counterpoise is a different animal. Now the return path exists by design, so the choke can be used to keep the rest of the feedline out of the antenna.

An end-fed off-center system, where a short section of shield or a dedicated conductor is intentionally allowed to participate up to a choke, sits somewhere in between. The choke is not an afterthought there ... it is the boundary marker between the radiating or current-carrying part of the structure and the quiet feedline you want arriving at the shack.

A classic two-terminal off-center-fed dipole is different again. It is still a dipole. Its main need is not “finding a missing half,” but preventing the feedline from becoming an unwanted third conductor. That is why a proper 1:1 current choke is usually the cleaner answer there.

Noise Floor Is Not the Same as Antenna Gain

One of the most helpful parts of DC4KU’s work is that he separated “the antenna receives signals” from “the system receives avoidable local noise.” When he added a proper choke in the right configuration, the receiver noise floor dropped markedly. That does not mean the antenna suddenly became less sensitive. It means the feedline stopped acting like a convenient pickup probe for house wiring and nearby electronic junk.

This is why a badly behaved EFHW is often described as “noisy” when the real culprit is the feedline acting as an auxiliary receive antenna in exactly the wrong place.

The Transformer Is Not a Lossless Decoration

Another useful reminder from DC4KU is that the transformer itself has real loss. In his example, the measured insertion loss of the transformer at 7.1 MHz was about 1.2 dB. That is not catastrophic, but it is also not zero. On the lower bands, where currents are high and ferrite stress is real, pretending the transformer is ideal leads to over-optimistic thinking.

This matters because many EFHW myths stack on top of each other: “the transformer is wideband,” “the feedline is not part of the antenna,” “a ground stake fixes everything,” and “the noise is just how end-feds are.” Once you measure instead of assuming, most of that story collapses.

A Note on Capacitors and Transformer Design

Many commercial EFHW designs rely on galvanically coupled transformers and fixed capacitor tuning to force a nicer-looking SWR curve. That does not automatically mean the antenna system is clean, quiet, or isolated.

RF.Guru EFHW transformer designs avoid using capacitors as a cosmetic cure for wideband mismatch. Capacitors are only useful when they solve a specific measured problem in a controlled design. They should not be used to hide feedline participation or make a transformer look more broadband than it really is.

A better-looking SWR curve is not the same as better common-mode control.

Practical Conclusions

• An EFHW can radiate acceptably while still having an unhealthy amount of feedline participation.
• A ground lead is not the same thing as controlled current routing.
• A short 0.05λ return section makes sense only when you also decide where that current must stop.
• A real common-mode choke is most useful when it separates the intentional RF structure from the rest of the station wiring.
• If your EFHW is “noisy,” do not blame the wire first ... blame the feedline until proven otherwise.
• Off-center-fed dipoles and end-fed systems are not interchangeable cases when discussing choke placement.
• SWR alone does not prove that the feedline is isolated.

“Etwas strahlt immer. Die Frage ist nur: was?”
— Adapted from DC4KU

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