EFHW Return Current: Why “Voltage-Fed” Does Not Mean Loss-Free

The claim sounds tidy: “An EFHW is voltage-fed, so return current is small, ground loss is minimal and about five per cent of a wavelength is enough counterpoise.” It bundles four different questions into one slogan. The feedpoint impedance, transformer currents, environmental return path and dissipative loss must be examined separately.

“Voltage-Fed” Describes a Port Condition

At one instant and frequency, the end of a resonant half-wave wire can present a high complex impedance. Relative to a low-impedance centre feed, the differential port current can be smaller and the terminal voltage larger for the same accepted power. That is all the label voltage-fed establishes.

A real EFHW feedpoint is frequency-, geometry- and environment-dependent and usually reactive away from a particular resonance. The transformer adds magnetising current, leakage, winding resistance, core loss, capacitance and electrical delay. Its primary and secondary currents are related by the actual network and terminations; the wire-terminal current is not automatically the same as exterior-coax or counterpoise current.

Current Still Has to Complete a Loop

“One wire” is a mechanical description, not a complete circuit. Charge that moves onto the radiator is accompanied by current and displacement-current closure through some combination of:

• a deliberate counterpoise wire or radial structure;
• a chosen section of coaxial-shield exterior before a choke;
• the mast, support, equipment enclosure, protective bonding or attached control cables;
• capacitance to soil, buildings, vegetation and nearby conductors; and
• the transformer’s interwinding and enclosure capacitances.

These branches divide current according to their complex impedances. The sum closes the electromagnetic circuit, but the current in one branch need not equal the differential current at the high-impedance wire terminal.

Roy Lewallen, W7EL, separates wanted coaxial transmission-line current from the additional current that can flow on the outside of the shield in Baluns: What They Do and How They Do It. The centre conductor and inside of the shield carry the differential mode. The outside of the shield is another conductor and can become an intentional or accidental antenna branch.

A Counterpoise Is a Designed Return Conductor

A counterpoise is not a charm that makes current disappear. It is a conductor or conductor system intended to provide part of the RF return. Its impedance depends on length, shape, height, route, termination, frequency and coupling to everything around it.

A five-per-cent-of-wavelength wire can be a useful experimental starting point in a named installation. It is not a universal optimum. On a multiband EFHW, the same physical wire has a different electrical length and environment on every band. It may carry substantial current on one frequency, little current on another, or interact with the coax and mast to create a new resonance.

A separate counterpoise is therefore not always mandatory. A deliberate coax-exterior section can perform that job when its route and terminating choke are part of the design. Conversely, an installation with no visible counterpoise has not proved that return current is negligible; it may have distributed the return over the feedline, station and environment.

Ground Loss Is Real Power, Not a Voltage Label

Ground loss means real power dissipated in lossy soil or another material because the antenna’s fields and currents couple to it. It is not determined by port current alone. A high-voltage region can create strong electric-field coupling, while a high-current region can drive magnetic-field and conductive loss. Geometry, height, ground conductivity and permittivity, conductor orientation and current phase all matter.

That is why neither “high-voltage feeds always lose more” nor “low feedpoint current means negligible ground loss” is defensible as a universal rule. The LLNL NEC-5 validation manual demonstrates that calculated antenna efficiency and impedance can be sensitive to ground conductivity, permittivity and radial geometry even for a carefully defined model. A real EFHW adds more conductors and less certain boundaries.

Count loss at one consistent boundary:

Current on a coax exterior is not automatically loss. It can radiate, alter the pattern, couple local noise or carry power into a lossy route. The measurement must distinguish those outcomes.

The Transformer Does Not Define the Return Path

A turns ratio predicts an impedance ratio only for an ideal transformer. For example, a nominal 49:1 device would ideally map 2450 + j0 Ω to 50 + j0 Ω. A real EFHW rarely presents that one resistive value across all operating frequencies, and a real transformer does not preserve the ideal relationship without loss and parasitic effects.

Choose the transformation function from the measured complex load and test it across frequency, voltage, current, mismatch, waveform, duty cycle and temperature. Choose common-mode suppression from the separate installed return-path problem. A transformer’s good input SWR does not prove low loss, correct balance or controlled shield current.

Joeri’s practical product-neutral default remains explicit: where the installed feed is deliberately unbalanced, use an UNUN for the required differential impedance transformation and a separately characterised choke at the intended end of the return branch. A suitable current balun remains valid where the installed load is genuinely balanced. One component may combine functions only when both have been demonstrated.

Choke Placement Defines a Boundary

Putting a choke directly at an end-fed transformer can reduce exterior-coax participation there. If no other deliberate return conductor exists, it can also change the feedpoint impedance and force the return through capacitance or other structures. That is a different antenna system, not automatically a better one.

If a section of coax exterior is the intended return conductor, place the first effective choke at the designed end of that section and verify the current on both sides. There is no universal 0.05- or 0.15-wavelength distance. The shield-exterior mode sees its route, jacket, soil, mast, nearby conductors and terminations—not simply the internal coax velocity factor.

The choke itself has a frequency-dependent complex impedance ZCM = RCM + jXCM. Both parts affect current; the resistive part dissipates real common-mode power and can heat. Small-signal impedance is not a power rating. Voltage, current, winding capacitance, ferrite nonlinearity, differential loss, ambient temperature and duty cycle all require qualification.

Measure More Than SWR

SWR answers a useful but narrow question about reflection at a stated plane. Changing counterpoise length, coax route or choke position can change the measured impedance because the current system changed. A lower SWR does not by itself show that loss fell or that more power went in the desired direction.

Keysight’s field cable-and-antenna measurement guidance separates return loss/VSWR from insertion loss and stresses calibration at the intended test plane. Use that same discipline for an EFHW: record the exact configuration and do not compare sweeps made through different uncharacterised cable paths as if they were the same measurement.

1. Draw every conductor. Include radiator, transformer, counterpoise, coax exterior, mast, equipment, bonds and nearby parallel cables.
2. Declare the intended return. State which conductor is meant to carry current and where that participation should end.
3. Mark reference planes. Record where impedance, loss, voltage, current and temperature are measured.
4. Save vector impedance. Keep resistance and reactance across every operating band, not only SWR minima.
5. Map exterior current. Use a calibrated current probe at repeatable positions along coax, counterpoise, mast and other possible branches.
6. Change one variable. Move one choke, reroute one cable or change one counterpoise dimension, then restore the baseline and repeat.
7. Check the desired result. Compare accepted power, field or pattern, receive SNR and station symptoms separately.
8. Qualify operating stress. Test the transformer, choke, cable and connectors at representative power, mismatch and duty cycle until temperature stabilises.

ITU-T K.37 (01/2024) treats common-mode mitigation as a system of cabling, balance, earthing, bonding, shielding and filtering choices. The lesson here is not to copy its telecommunications installation directly, but to retain the system view: one ferrite part cannot define every current path around it.

What Common Observations Actually Prove

Primary and Authoritative Sources

• Roy Lewallen, W7EL — Baluns: What They Do and How They Do It: primary explanation of differential coax current, shield-exterior imbalance current and path-dependent division.
• Lawrence Livermore National Laboratory — NEC-5 Validation Manual: official modelling validation, including sensitivity to ground parameters and radial geometry.
• Keysight — Techniques for Precise Cable and Antenna Measurements in the Field: calibration, insertion loss, return loss, VSWR and test-plane boundaries.
• ITU-T K.37 (01/2024): in-force system-level common-mode and EMC mitigation guidance.

Joeri’s Bottom Line

Low current at one high-impedance terminal does not make the rest of the circuit vanish. It does not tell us how current divides among counterpoise, coax, mast and environment, and it does not quantify real power lost in the transformer, feed line, ground or nearby materials.

An EFHW can be an effective antenna when its transformer, return path and common-mode boundary are engineered as one installed system. Stop asking whether five per cent of a wavelength is “enough” in the abstract. Draw the path, measure the current at more than one point, account for power at declared planes and qualify the hardware under the real load.

Questions, antenna-factor records or height trials to share? Contact RF.Guru.