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
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.
Werner Schnorrenberg, DC4KU, did not just repeat forum folklore. He measured antenna current, common-mode current on the feedline, and receiver noise floor for several EFHW configurations. His work is valuable because it shows that “the antenna still works” and “the feedline is no longer part of the problem” are two very different statements.
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.
In DC4KU’s 7.1 MHz / 10 W example, the “standard” HyEndFed configuration showed about 320 mA of antenna current and about 40 mA of common-mode current on the coax. In other words, the antenna was radiating ... but the feedline was participating too. That is precisely why a bare EFHW can seem acceptable on air while still causing extra noise pickup, shack RF, or unstable behavior.
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.
With a simple earth connection at the transformer, the coax still carried substantial common-mode current, and some current now also flowed in the ground lead. In other words, the return path was spread out, not truly controlled. That is why “just ground it” is often a disappointing answer for end-fed antennas installed near houses and domestic noise sources.
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 still prefers to flow on the outside of the coax unless you raise the impedance of that path with a proper choke.
A short pigtail or a short coax section ahead of the choke is not “random extra wire.” It is an intentional return conductor. The choke then marks the point beyond which that current should not continue. In other words, the choke is not there to kill the whole return path ... it is there to stop the return path from extending all the way back to the station.
That distinction is exactly why choke placement right at the box and choke placement a short distance down the line can produce very different results. They are solving different versions of the same problem.
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, and band.
| Configuration | Coax common-mode current | Other return current | Observed receiver noise floor | What it means |
|---|---|---|---|---|
| Standard HyEndFed | About 40 mA | None intentionally defined | About -75 dBm | The antenna works, but the feedline is clearly part of the antenna system. |
| Transformer grounded | About 30 mA | About 20 mA in the ground lead | About -75 dBm | Grounding split the current; it did not truly quiet the installation. |
| 0.05λ pigtail only | About 20 mA | About 20 mA in the pigtail | About -75 dBm | The pigtail helps, but the coax is still involved. |
| 0.05λ pigtail + real choke | About 5 mA | About 35 mA in the pigtail | About -93 dBm | This is much closer to controlled current routing. |
| 0.05λ coax section + real choke | Essentially 0 mA behind the choke | About 40 mA in the intentional coax section ahead of the choke | About -93 dBm | The coax section ahead of the choke is now the intended return conductor; the rest of the line is “cold.” |
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.
DC4KU’s paper is about EFHW behavior, but the deeper lesson generalizes well: the right choke position depends on whether the feedline segment before the choke is accidental or intentional. If that segment is part of the return-path design, the choke belongs after it. If it is not supposed to participate at all, the choke belongs where it can stop participation immediately.
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.
A choke does not magically create gain. What it often does create is honesty. It removes unwanted feedline pickup, lowers the local noise floor, and lets the actual antenna do the listening instead of your shack wiring.
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.
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.
Mini-FAQ
- Does an EFHW always need a counterpoise? In practice, yes ... the only real question is whether you deliberately provide it or allow the coax and surroundings to become it.
- Why is 0.05λ mentioned so often? Because it is a practical minimum length for a short return conductor in many end-fed half-wave setups, but it works best when paired with correct choke placement.
- Can I just ground the transformer box? Not as a universal cure. It may simply create another RF path instead of removing common-mode current from the feedline.
- Why does a choke sometimes go away from the feedpoint? Because the short section before the choke may be intentionally acting as the return path. The choke then marks where that current must stop.
- Why can an off-center-fed dipole use a different choke strategy? Because it is a two-terminal radiator. Its issue is usually keeping the feedline from becoming an unwanted third conductor, not inventing a missing return conductor.
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EFHW Coax Lengths and Choke Placement: Lessons from DC4KU
Reference: Werner Schnorrenberg – DC4KU, “Endgespeister Dipol mit Gegengewicht und Mantelwellensperre” (2019). Field-verified by RF.Guru designs and measurements.
Many common-mode and noise issues in end-fed antennas can be traced back to one simple oversight: the coaxial feedline is part of the antenna unless you stop it.
DC4KU’s 2019 work shows that improperly managed end-fed systems radiate from the coax — and not just during TX. They also pick up household noise, often adding 3+ S-units of baseline QRM, especially on low bands.
No Capacitors
While many commercial EFHW designs rely on galvanically coupled transformers and fixed capacitor tuning, RF.Guru EFHWs use a fully decoupled winding architecture and no capacitors.
Recommended Coax Lengths Before the Choke
Per DC4KU, using FlexBury 7 mm coax (VF = 0.86). Place the choke at ~0.05 λ from the feedpoint — the coax segment acts as a functional counterpoise.
| Band | Wavelength (λ) | 0.05 λ (electrical) | Physical length (VF=0.86) |
|---|---|---|---|
| 160 m | 187 m | 9.35 m | ≈8 m |
| 80 m | 83 m | 4.15 m | ≈4 m |
| 40 m | 41 m | 2.05 m | ≈2 m |
| 20 m | 20 m | 1.00 m | ≈1 m |
| 10 m | 10 m | 0.50 m | ≈0.5 m |
Can You Roll Up Excess Coax?
Yes — and often you should. As long as you’ve implemented:
- A proper RVS ground rod
- A counterpoise wire (2–4 m or more, depending on band)
…you can loosely roll the coax in coils or bury it without performance loss. Just avoid tight coupling near metal objects.
What Happens If Coax Is Too Short?
DC4KU’s lab data demonstrates how coax length and choke placement affect currents and noise:
| Setup | Common-mode current | RX noise floor (80 m) |
|---|---|---|
| No choke, no counterpoise | ~40 mA | ≈−75 dBm (S8) |
| Counterpoise only | ~20 mA | ≈−75 dBm (S8) |
| Choke at feedpoint + counterpoise | ~5 mA | ≈−93 dBm (S5) |
| Choke 0.05 λ before feedpoint | ~0 mA | ≈−93 dBm (S5) |
Even with a perfect transformer, RF finds its way back — usually via the coax shield, router, PC, or even the neighbor’s lamp.
Best Practice (RF.Guru EFHWs)
- Use enough coax before your choke — don’t skimp.
- Always install a proper common-mode choke.
- Add a 1–4 m counterpoise wire terminated on an RVS rod.
- Roll up extra coax if needed; no need for perfect straight runs.
Don’t Trust SWR Alone
Even with a “perfect” 1.1:1 SWR, your shack may still radiate — especially if:
- Coax is too short
- Choke is placed too close to the transformer
- No counterpoise or RVS rod is used
Our transformers avoid the high self-capacitance issues DC4KU flagged, and only use capacitors where beneficial.
Final Word
Your EFHW isn’t just about the wire and transformer — it’s a system, and the coax is part of it until you stop it.
“Etwas strahlt immer. Die Frage ist nur: was?”
— Adapted from DC4KU
Mini-FAQ
- How much coax before the choke? — About 0.05 λ, adjusted for coax VF.
- Can coax be coiled? — Yes, loosely or underground, if counterpoise and ground are provided.
- Does low SWR mean no common-mode? — No — SWR doesn’t guarantee isolation.
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Questions or experiences to share? Contact RF.Guru.