Your EFHW Isn’t Noisy — Your Feedline Is
Your EFHW Isn’t Noisy — Your Feedline Is
An EFHW is not born with a special appetite for noise. When one installation sounds noisier than another, trace the field, current path, antenna pattern and receiver response before blaming the feed method.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
“EFHWs are noisy” is one of those statements that sounds convincing after a quick antenna swap. The S-meter rises, the waterfall fills in and the end-fed wire gets the blame. But the receiver is showing the complete installed system: antenna pattern, location, loss, feed line, exterior current, connected equipment and receiver state. The label on the antenna cannot separate those contributions.
Short version: a noisy EFHW installation often has a feed line or station path participating in reception—but often is not always. Measure the exterior current and compare signal-to-noise ratio before prescribing a choke.
“Noise” Has More Than One Source
At an HF receiver input, the indicated background can contain several things at once:
- Natural external noise: atmospheric discharges, galactic and solar contributions, and thermal radiation from the environment.
- Man-made external noise: unintended emissions from power electronics, wiring, networks, motors, lighting and many other devices.
- Antenna-system thermal noise: loss in conductors, ground, transformer, feed line and other resistive parts contributes noise at their physical temperature.
- Receiver noise and artefacts: front-end noise, overload, intermodulation, ADC clipping, gain control and display processing can all change what the operator sees.
A passive antenna does not manufacture switch-mode hash, but it does receive external fields and its losses contribute thermal noise. More importantly, two antennas seldom sample the same field distribution. Different height, shape, orientation and current distribution can hear different directions and polarizations even when both feed lines are perfectly controlled.
ITU-R P.372-17 explicitly limits its reference noise data to noise received through the reference antenna and feeder. It excludes noise entering through other conducting structures or through inadequate feeder screening or balance. That distinction is exactly the one an antenna comparison must preserve.
The Coax Can Carry Two Different Current Systems
In the intended coaxial TEM mode, current on the centre conductor is accompanied by equal-and-opposite current on the inside surface of the shield. Most of the field is confined between those conductors. This is the signal path the coax was built to carry.
The outside surface of the shield can support another current with respect to the antenna, equipment, nearby structures and the wider environment. In amateur-station language, that exterior-shield current is the practical common-mode problem. It is not the inner-surface return current of the coaxial TEM mode.
| Current system | Primary path | What sets it |
|---|---|---|
| Internal coaxial mode | Centre conductor and inner shield surface | Coax geometry, differential load, connectors and mismatch |
| Exterior/common mode | Outer shield surface and environmental return paths | Installed geometry, asymmetry, choke impedance, counterpoise, bonding and surroundings |
If the exterior mode is appreciable, the long coax run can become another receiving conductor. A cable routed past chargers, Ethernet leads, LED wiring and house mains may then deliver locally coupled noise to the antenna terminal or receiver enclosure. On transmit, the same path can alter the pattern, move the impedance, couple RF into station wiring or create accessible RF voltage.
ITU-T K.37 (2024) describes a common-mode choke as added impedance in the common-mode circuit and stresses that the achieved current reduction depends on the original circuit impedance and system balance. A choke is therefore part of an installed network, not a universal number of turns or a guaranteed cure.
Why an EFHW Can Recruit the Feed Line
An end-fed wire and matching transformer do not form an isolated one-terminal source. The RF circuit closes through some combination of:
- a dedicated counterpoise or short return conductor;
- the exterior of the feed line;
- equipment enclosures, bonding and connected cables;
- nearby conductive structures; and
- distributed displacement current to the environment.
Which paths dominate depends on frequency and geometry. If an installation offers no low-enough-impedance local return, the coax exterior can become a substantial part of the antenna system. That does not make every EFHW noisy, nor does it prove that all coax-exterior current is harmful. It says the actual antenna boundary must be measured rather than assumed.
A centre-fed dipole is not automatically exempt. Unequal leg geometry, nearby metal, a sloping feed line, equipment connections or an inadequate feedpoint choke can also produce exterior current and local-noise pickup. The useful comparison is not “EFHW versus dipole” in the abstract; it is one complete installation versus another.
A Choke Changes the Circuit
A common-mode choke presents complex impedance to the exterior-current path. Its effect depends on frequency, placement, current level, temperature and the other available return paths. Installing one can reduce local-noise coupling, but it can also change terminal impedance and current distribution because it changes the antenna system itself.
That is why fixed prescriptions—always at the transformer, always at the shack, always two chokes—are not engineering. A defensible placement process is:
- map exterior current along the feed line with a suitable current probe;
- identify the suspected noise source and coupling route;
- place a characterized choke where it interrupts that path without creating an unsafe voltage or unwanted return-path change;
- repeat the current and SNR measurements on every operating band; and
- check transmit current, temperature, pattern symptoms and RF-exposure/accessibility conditions at the intended power and duty cycle.
The ARRL current-probe article by Larry Lamano, WAØQZY illustrates measurement around the complete coax so the probe responds to net exterior current. Measuring only the centre conductor or only one shield surface answers a different question.
A Counterpoise Is a Circuit Element, Not a Magic Length
A dedicated counterpoise can give return current a more deliberate path on the antenna side of a choke. But its impedance depends on length, height, route, insulation, nearby soil and structures, and frequency. A value such as 0.05 wavelength may be a starting geometry for an experiment; it is not a broadband law and does not guarantee a non-resonant or low-loss return.
The counterpoise must also remain distinct from protective earth and lightning bonding. Those systems have safety functions that must not be defeated to obtain a convenient RF trace. If the choke or counterpoise changes SWR, that is evidence that the installed current distribution changed—not proof that radiation efficiency improved or worsened.
Compare SNR, Not S-Meter Height
A lower noise floor is useful only if the wanted signal does not fall by the same amount. For each antenna, record wanted-signal power and nearby background power in the same receiver bandwidth:
SNRdB = Psignal,dB − Pnoise,dB
Keep frequency, bandwidth, detector, preamplifier, attenuation, AGC and display averaging fixed. Use a stable reference signal where possible, switch rapidly or measure simultaneously, repeat the sequence as A/B/A, and record propagation and local-device states. If both signal and noise drop 6 dB, the apparent “quieting” produced no SNR gain.
ITU-R SM.2093 gives a reproducible framework for indoor radio-environment measurements, including receiver settings, antenna, measurement positions, time sampling and post-processing. ITU-R SM.2055 likewise treats noise as power at a declared antenna connector and emphasizes repeatable antenna-system and feeder conditions.
Use Controlled Changes to Find the Path
| Controlled change | What it can indicate | What it cannot prove alone |
|---|---|---|
| Clamp a characterized choke at a measured location | Sensitivity to that exterior-current path | That the EFHW was inherently noisy or that all exterior current was removed |
| Move or reroute the coax while keeping antenna geometry fixed | Coupling or current-distribution sensitivity | The exact source or coupling mode |
| Terminate the receiver at the antenna-plane reference | Noise entering after that plane | Noise the antenna would receive in service |
| Switch suspected local devices through A/B/A states | Source correlation | The complete transfer path without current or field evidence |
| Add receiver attenuation or preselection | Overload or internal mixing sensitivity | That the external antenna noise disappeared |
The practical references from W8JI on common-mode current, W8JI on common-mode noise, AB9IL on receive-noise choking and PA9X on EFHW counterpoise layouts are useful sources of experiments. Treat their dimensions and placements as configurations to test, not universal installation constants.
The Direct Answer
An EFHW is not inherently noisier than a dipole. A particular EFHW can sound noisier because its pattern samples different noise, because its feedpoint is closer to the source, because the coax exterior or station wiring participates, because losses and gain differ, or because the receiver is responding nonlinearly.
So keep the memorable title, but use it as a diagnostic hypothesis: if your EFHW sounds noisy, investigate the feed line first—then prove the path with current and SNR measurements. The result may be a better choke or counterpoise. It may instead be a different antenna location, pattern, cable route, source fix or receiver setting. The measurement decides.
Mini-FAQ
- Are EFHWs inherently noisier than dipoles? — No. The result depends on pattern, placement, loss, exterior current, local coupling and receiver state.
- Does a choke automatically make an EFHW quieter? — No. It changes one exterior-current path; measure current and SNR before and after installation on each band.
- Where should the choke go? — At a location selected from the installed current path, return geometry, choke impedance, voltage stress and operating bands—not from a universal distance.
- Does every EFHW need the same counterpoise length? — No. Counterpoise impedance depends on frequency, geometry, height, surroundings and the other return paths.
- Why is a lower noise floor not enough? — Because signal may fall by the same amount. Compare signal-to-noise ratio with fixed receiver settings.
- Does low SWR prove the feed line is quiet? — No. SWR describes reflection at one plane and does not measure exterior-shield current or local-noise coupling.