Coax Before the Choke: Define the EFHW Return Path
Coax Before the Choke: Define the EFHW Return Path
Werner Schnorrenberg, DC4KU, measured what happens when an end-fed antenna uses a pigtail or part of the coax exterior as its return conductor. His experiment is valuable because it turns an invisible current path into something we can discuss and measure—but it does not make one length correct for every installation.
A choke at the transformer and a choke farther down the coax solve different circuits. If a deliberate section of shield exterior is the end-fed antenna's return conductor, the choke belongs at the far end of that section. If the coax exterior is not meant to participate, the boundary belongs close to the feedpoint. The right position is the one the installed current map supports.
My practical position: decide which conductor completes the RF circuit, place a separately specified choke where that conductor should stop, and verify the result on every operating band. “Coax before the choke” can be deliberate antenna structure. It is not a cable-length trick for improving an SWR number.
The Coax Can Have Two Electrical Jobs
In the wanted coaxial transmission-line mode, current travels on the centre conductor and returns on the shield's inner surface. Those opposing fields are substantially confined by the cable. Current on the shield's outside is a different mode. Its return may involve a counterpoise, soil, a mast, building conductors, station wiring and capacitance to the surroundings.
An end-fed wire needs a complete RF circuit even when the wire-side feedpoint impedance is high. If no deliberate return conductor is provided, the coax exterior and whatever it couples to can become part of that circuit. Moving the cable, changing its length or connecting different station equipment can then change current distribution, impedance, received noise and the installed pattern.
That participation is not automatically a defect. A known section of shield exterior can be used intentionally. The mistake is leaving its length and termination undefined while pretending the coax is only a feedline.
The First Choke Is a Current Boundary
A common-mode choke adds impedance in the exterior-current path while passing the wanted differential signal inside the coax. When the exterior section before it is intentional, the choke marks the end of that chosen return conductor. The cable beyond the choke is supposed to remain outside that branch of the antenna circuit.
“Supposed to” matters. Choke impedance is complex and frequency-dependent. Core material, winding geometry, parasitic capacitance, the connected common-mode load, temperature and drive level all affect its behaviour. A turns count, core label or impedance at one frequency does not prove the boundary is effective across a multiband antenna.
Measure current around the complete coax, not one conductor inside it, at several repeatable positions before and after the choke. Repeat on every required band. A single measurement at the radio can miss a current maximum farther up the line.
What DC4KU's Experiment Actually Shows
DC4KU's 2019 paper compared several arrangements around a HyEndFed-style antenna. Table 1 reports antenna and exterior-current measurements at 7.1 MHz with 10 W, while the receiver-noise entries were recorded at 3.7 MHz in his station and noise environment. Three configurations make the return-path point especially clearly.
| DC4KU configuration | Reported antenna current at 7.1 MHz | Reported exterior current beyond the intended boundary | Reported 3.7 MHz receiver noise |
|---|---|---|---|
| Standard autotransformer arrangement, no defined external counterpoise | About 320 mA | About 40 mA on the feedline | About −75 dBm |
| 0.05λ pigtail plus the measured choke | About 320 mA | About 5 mA on the feedline; about 35 mA in the pigtail | About −93 dBm |
| 0.05λ coax-exterior section plus the measured choke | About 320 mA | Reported as 0 mA beyond the choke; about 40 mA in the intended coax section | About −93 dBm |
Those results support the article's central narrative: the antenna can still carry substantial wire current while the feedline exterior is participating, and a deliberately terminated return section can greatly change current beyond the choke and local-noise pickup in that particular installation.
They do not establish that 0.05λ is the optimum return length for every transformer, route, height, soil, building or band. They also do not turn the reported noise-floor change into antenna gain, radiation efficiency or a universal noise reduction. It is one carefully documented configuration and a strong reason to measure our own.
Why 0.05λ Is a Test Case, Not a Law
A short return conductor can be enough to complete an end-fed circuit without allowing the entire feedline to become uncontrolled antenna structure. That is why 0.05λ appears in end-fed discussions and why DC4KU tested it. But “enough” depends on the impedance of every available return branch and on the current distribution of the complete installed system.
A multiband installation makes a fixed fraction especially fragile. One physical section represents a different electrical condition on 80, 40, 20, 15 and 10 metres. Its coupling to soil, a mast and nearby wiring also changes with frequency. A length that gives a convenient result on one band can land near a current maximum, minimum or resonance on another.
Use 0.05λ only as a documented experimental starting point when the configuration resembles the experiment. Then move the boundary, map current and restore the original setup in an A/B/A comparison. The installed evidence decides whether the section is long enough, too long or simply routed badly.
Do Not Apply the Cable's Velocity Factor to Its Exterior
A coax datasheet's velocity factor describes propagation in the specified internal transmission-line mode. It is appropriate when calculating the differential electrical length between two coaxial reference planes.
The exterior-current path sees a different electromagnetic environment. Jacket, height, soil, wet surfaces, mast bonding, nearby conductors and common-mode terminations can all influence its effective propagation and impedance. Multiplying a free-space counterpoise length by the coax's internal velocity factor therefore does not produce a universal physical length for the outside of the shield.
This is also why neatly calculating a quarter-wave current maximum on paper cannot replace a current scan on the installed cable. The structure around the shield is part of that calculation whether it appears on the sketch or not.
Transformation and Choking Are Separate Jobs
An UNUN can transform between intentionally unbalanced ports. A common-mode choke establishes impedance in the separate exterior-current path. Combining both functions in one enclosure does not prove that each function works over the same frequency, complex load, voltage, current, power, duty cycle and temperature range.
For practical end-fed work, I prefer to treat those jobs separately. That lets me characterise transformer transmission and stress with representative loads, then choose and place the choke from the actual exterior-current path. A favourable SWR curve is not evidence that either part is low-loss or that feedline current is controlled.
Likewise, a ground lug, earth stake or station bond is not automatically the missing RF return. Adding a conductor changes the network; current divides according to the impedances of all available paths. Protective earthing and lightning bonding remain safety functions and must not be removed or repurposed merely to obtain a preferred RF trace.
Keep SWR and the Current Boundary Separate
A VNA reports reflection at its calibrated reference plane. Adding cable between that plane and a mismatched load changes the displayed resistance and reactance through ordinary transmission-line transformation. Real cable also attenuates the reflected wave, so a longer line can make source-end SWR look better while delivering less power to the antenna.
Moving the first choke can cause a second kind of change: it alters the length and termination of the coax-exterior branch. If both the SWR trace and exterior-current map change, the installed antenna has changed as well as the feedline boundary.
| Observation | What it may indicate | What to check next |
|---|---|---|
| R + jX changes after adding cable, but the exterior-current map and route remain stable | Differential line transformation between reference planes | Characterise or de-embed the cable and compare at the same calibrated plane |
| Source-end SWR improves as cable gets longer | Transformation, round-trip attenuation, or both | Measure line loss; do not infer better radiation from the source-end trace |
| Moving the choke changes current, impedance and received noise | The exterior section is materially involved in the antenna and local coupling | Repeat the current scan and restore the original geometry in an A/B/A test |
| Current is low at the radio but high farther up the feedline | The station-end point is near a current minimum, not necessarily isolated | Scan multiple marked positions along the complete cable |
| One band improves while another gets worse | Different multiband current distributions and choke impedance | Repeat impedance, current and thermal checks band by band |
| A low SWR accompanies transformer or choke heating | Accepted power may be turning into component loss | Measure transmission with a representative load and qualify at operating duty cycle |
A Repeatable Way to Choose the Length
- Draw the whole conductor network. Include the wire, transformer ports, deliberate counterpoise, coax exterior to the first choke, mast, bonds, equipment and control or mains connections.
- Declare the intended return. State whether the shield exterior is meant to participate and exactly where that participation should end.
- Mark the physical route. Record height, proximity to metal and soil, bends, coils and every connection. Route changes are circuit changes.
- Calibrate at a named plane. Save frequency and complex R + jX or S11, not only minimum SWR.
- Map exterior current. Use the same calibrated RF current probe and marked positions before and after the choke on each band.
- Change one variable. Move the choke or change the intentional section while keeping wire geometry, route and station connections fixed.
- Restore the baseline. Repeat A/B/A so connector repeatability, weather and accidental movement do not masquerade as an improvement.
- Check the consequences separately. Measure line and transformer loss, choke impedance, component temperature, station RF and received SNR; use field or pattern evidence for radiation claims.
Never connect an analyser to a live transmitter. Respect the instrument's maximum input, discharge static safely, keep people clear during transmit tests and follow the required RF-exposure, grounding, bonding and overhead-line rules for the site.
What About Excess Coax?
Do not hide excess cable by changing the intentional return section without documenting it. Before the choke, the route and length can affect exterior current and coupling. After an effective choke, extra cable mainly belongs to the differential feedline—but only after current measurements show that the boundary is working.
A random coil of coax is not automatically a broadband choke. Its common-mode impedance can be narrowband and strongly affected by turn-to-turn capacitance and construction. Use measured choke data over the required bands, and repeat the installed current scan after the cable is dressed.
Primary Sources and Measurement Context
- Werner Schnorrenberg, DC4KU, Endgespeister Dipol mit Gegengewicht und Mantelwellensperre—the 2019 antenna-current, feedline-current and receiver-noise experiment that prompted this article.
- ARRL, Common-Mode Chokes—current measurement, frequency-dependent choke impedance and self-resonance.
- Keysight, S-Parameter Design—travelling waves, reflection coefficient and transmission-line behaviour.
- Keysight, Techniques for Precise Cable and Antenna Measurements in the Field—calibration, return loss, VSWR and cable loss.
- Bockelman and Eisenstadt, “Combined Differential and Common-Mode Scattering Parameters”—formal separation of differential and common-mode network behaviour.
- TDK, Measuring Common-Mode and Differential-Mode Choke Impedance—different fixtures for the two modes.
- Tom Rauch, W8JI, Common-Mode Current—installed current paths and the location-dependent effect of suppression.
Joeri's Bottom Line
The coax before the first choke may be part of the antenna. If I choose it as the return conductor, I say so, keep its route repeatable and place the choke where that conductor should end. Then I prove the boundary with a current map on every band.
DC4KU's 0.05λ experiment is useful evidence, not a universal cutting table. The installed antenna decides the required length through its current distribution, surroundings, common-mode terminations and operating frequency. Measure that circuit, not the folklore.
Mini-FAQ
- How much coax should be left before the first EFHW choke? Use enough to form the intended return conductor in the installed system, then locate the boundary from band-by-band exterior-current measurements. There is no universal physical length.
- Is 0.05λ always the correct return length? No. It is a documented experimental starting point, not a universal optimum across different bands, routes, transformers, surroundings and common-mode loads.
- Should coax velocity factor be used to calculate the exterior return section? Not as a general rule. Datasheet velocity factor describes the internal coaxial mode; the shield-exterior path depends on its installed electromagnetic environment.
- Why can moving the choke change SWR? Moving it can change the coax-exterior branch of the antenna, while the coax between measurement and load also transforms impedance. Record the reference plane and current map to separate the mechanisms.
- Can low SWR prove that the choke position is correct? No. SWR describes reflection at a stated plane. It does not prove low exterior current, low loss, safe component stress, good efficiency or a useful radiation pattern.
- Can excess coax be coiled after the choke? It can be dressed after a verified boundary, but a random coil is not automatically a broadband choke. Recheck exterior current after routing or coiling the cable.