Any Antenna Works. Efficiency Was the Question.
Any Antenna Works. Efficiency Was the Question.
Peter G3OJV makes a useful case against panicking over modest mismatch on a short HF coax run. That is one loss mechanism. It is not yet a measurement of the complete EFHW antenna system.
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.
A wet string can make a contact. A dummy load with a short lead can make a contact. A compromised EFHW can make many contacts. None of those observations answers the engineering question: what fraction of accepted power produces the intended far field, with the intended current distribution and pattern?
The source event: in EFHW Antenna Efficiency. Just how much loss is there in this popular antenna, Peter G3OJV of Waters & Stanton estimates losses in the coax and a generic 49:1 transformer, then argues that the practical total can remain modest. I agree with his narrow warning against treating a moderate SWR on short HF coax as a catastrophe.
Where I part company: a coax-loss calculation and a generic transformer curve do not establish total EFHW efficiency. The transformer must be tested with a representative complex load, the return path must be defined, common-mode current and temperature must be measured, and the radiated result needs a field or accepted-power reference.
What Peter Actually Argues
The video is worth watching in full because it is not simply an advertisement for a perfect antenna. Peter names coax loss, SWR, transformer loss, core size, operating power and installation quality. He also recommends a line isolator at the station end. The disagreement is about what those observations can prove when they are combined.
| Video point | What it establishes | What remains open |
|---|---|---|
| 00:14 — a half-wave wire on the lowest band can operate on higher modes. | End-fed half-wave wires can be useful multiband radiators. | Higher modes do not preserve the same feed impedance, current distribution, pattern or transformer load. |
| 03:40 — start with matched coax loss, then add the effect of roughly 2:1 VSWR. | This is a sensible way to estimate feedline attenuation for a named cable, frequency and length. | It does not include transformer dissipation, unintended outside-shield current, tuner loss, ground coupling or radiation efficiency. |
| 06:16 — the extra coax loss at 2:1 can be small. | Often true for a short, low-loss HF cable under the stated conditions. | A small incremental mismatch loss cannot be promoted to a total-system efficiency result. |
| 08:54 — use a generic FT240-43 transformer response. | A frequency-response curve can expose that the transformer has a finite useful band. | The curve needs topology, winding, power, temperature, fixture, reference planes and terminating impedance before it represents a particular EFHW installation. |
| 10:35 — the figures assume a good, resonant installation. | The result is conditional, not universal. | “Good” and “resonant” still need measurable definitions; a low input reactance or SWR does not identify where accepted power went. |
| 12:19 — larger or stacked cores can reduce loss and offer more margin. | Core volume, flux density and temperature matter. | More ferrite alone does not remove leakage inductance, winding capacitance, load reactance or the low-band/high-band design trade-off. |
| 12:34 — every multiband antenna loses something. | Every real system has loss and compromise. | Trap, conductor, tuner, line, ferrite, ground and common-mode losses are different quantities; they are not automatically a level playing field. |
| 13:15 — use a line isolator at the shack end. | A station-end choke can reduce current entering equipment and wiring. | It does not retroactively define the section of coax between feedpoint and choke; that section may already carry antenna return current. |
Mismatch Loss and Total Efficiency Answer Different Questions
Return loss and SWR describe reflection at a reference plane. Feedline attenuation describes real power dissipated in the cable as conductor and dielectric loss. The two interact, but neither quantity separates useful radiation from every other loss in the antenna system.
Keysight's official FieldFox cable-and-antenna measurement note makes the reference-plane issue explicit: insertion loss, return loss and VSWR are separate measurements, and cable loss can mask the antenna response. That is why a calculator result for a short coax run must stay a coax result.
A good match is not proof of good efficiency. A resistive loss can improve the match by absorbing energy. A tuner or compensation capacitor can move the input impedance without improving the far field. A coax segment can contribute radiation or pickup through outside-shield current while still carrying the wanted differential-mode wave internally.
The Transformer Does Not See a Fixed Resistor
The familiar 49:1 label is a nominal impedance transformation based on turns ratio. It is not a promise that a real wire presents a fixed 2,450 Ω resistance on every intended band. The load seen by the transformer changes with frequency, wire geometry, height, conductor diameter, nearby objects, soil coupling, the chosen return conductor and the coax route.
A two-port transformer measurement into defined resistive terminations is still useful. It can reveal insertion loss, amplitude response and resonance in that fixture. It cannot, by itself, reproduce a high-voltage, frequency-dependent and often reactive antenna load.
Fair-Rite's own broadband-transformer guidance divides response into low-, mid- and high-frequency regions. Low-frequency behaviour depends on magnetising inductance and core properties; high-frequency behaviour is increasingly affected by leakage inductance and winding capacitance. Core material, geometry, turns, winding layout, frequency, flux and temperature are therefore part of the result.
A defensible transformer statement names the conditions: topology and ratio, exact core material and stack, winding construction, frequency, source and load impedances including phase, drive waveform, duty cycle, ambient temperature, reference planes and measurement uncertainty.
Higher Modes Are Useful, Not Identical
A half-wave wire on the lowest operating band can support higher-order standing-wave modes. That is the real reason an EFHW can cover several amateur bands. It does not behave as the same half-wave antenna repeated at clean octaves.
On a higher mode, the wire contains more current maxima and minima. The radiation pattern develops additional lobes and nulls. The feed impedance and its reactive part change. End effect, bends, slope, proximity and the transformer or compensation network shift the frequencies at which the station sees a convenient match. Fifteen metres being useful on a nominal forty-metre EFHW is not an octave argument; it is a particular higher-mode and installation result.
This matters because a transformer that is comfortable at one band can encounter a less favourable magnitude and phase of load impedance on another. “It tunes” does not settle transformer heating, voltage stress or pattern.
A Capacitor Can Improve the Match Without Proving Efficiency
A shunt capacitor across the low-impedance side can be a legitimate compensation element. It can counter part of the transformer's high-frequency reactance and move an upper-band SWR minimum. That is useful engineering when the target, tolerance, voltage, current and loss are defined.
But it is not an efficiency certificate. The capacitor changes the input network. It may introduce circulating reactive current, and its ESR, voltage rating and temperature coefficient become part of the system. The resulting SWR can be better even when the far-field power is unchanged or another component is dissipating more power.
An End-Fed Wire Still Needs a Return Path
“End-fed” describes where the radiator is connected. It does not eliminate Kirchhoff's current continuity or the displacement-current path through the surrounding environment. If the installation does not define a return conductor or counterpoise, return current can occupy the outside of the coax shield, mast, equipment bonding, wiring, nearby structures and capacitance to ground.
A choke at the station may be valuable for station immunity and operator exposure, but the coax before that choke can still be part of the RF system. Moving the choke changes the current boundary. That can change pattern, received noise, feed impedance and loss. The appropriate position follows from the intended current path and measurements along the actual installation—not from a universal distance or a single SWR reading.
How to Test the Complete Question
There is no single bench measurement that turns an installed antenna into one trustworthy efficiency number across 80 through 10 metres. A useful evidence package combines several measurements:
- Define the antenna system. Record wire geometry, height, transformer, intentional return conductor, coax type and length, choke positions, tuner and nearby conductors.
- Set the reference planes. Calibrate the VNA or power measurement at named ports and remove fixture loss only when it has been characterised.
- Characterise the transformer. Measure complex input and transfer behaviour with representative resistive and reactive loads across the bands, at low level first.
- Measure common-mode current. Map outside-shield current at multiple positions without moving the cable between comparisons.
- Check thermal behaviour. Use stated power, waveform, duty cycle, key-down duration and ambient conditions. Temperature rise is evidence of dissipation, but converting it to watts requires a calibrated thermal model.
- Use accepted-power field comparison. Compare against a stable reference antenna at the same site and frequency, with controlled geometry, receiver linearity, propagation window and uncertainty.
- Repeat across bands and configurations. One favourable frequency cannot rate a multiband system.
High-voltage warning: an EFHW feedpoint and transformer can develop hazardous RF voltage. Temperature and common-mode tests must be arranged remotely, with the system de-energised before changing wiring or touching the antenna, feedline, transformer or instruments.
The Fair Conclusion
Peter's practical point survives: on many installations, the extra loss caused by moderate mismatch on a short, suitable HF coax run is small enough that buying a much larger cable will not transform the station.
The larger claim does not follow automatically. Total EFHW efficiency includes the wire's loss and current distribution, transformer dissipation under its actual complex load, the intentional or accidental return path, feedline attenuation, common-mode current, matching components and the surrounding installation.
“It works” is a valid operating report. “It is efficient” is a measurement claim. The difference is not antenna religion. It is simply the difference between an observed contact and a complete power balance.
Mini-FAQ
- Does this article say an EFHW cannot work well? No. It says contacts and SWR do not, by themselves, quantify efficiency or identify where power is dissipated.
- Is 2:1 SWR on a short HF coax run a major loss? Often it is only a modest additional cable loss, but the answer requires the exact cable, frequency, length, temperature and reference condition.
- Can a back-to-back transformer test prove installed EFHW efficiency? No. It characterises the paired transformers and fixture under the chosen terminations; the installed wire, reactive load, return path and field result are not reproduced automatically.
- Does a shunt capacitor make an EFHW inefficient? Not necessarily. It can be a valid compensation element, but an improved match does not prove improved radiation efficiency.
- Where should the common-mode choke go? Place it to enforce the intended current boundary, then verify outside-shield current and station behaviour. A station-end choke and a feedpoint choke solve different parts of the system.
- What evidence would support an efficiency claim? Defined reference planes, transformer tests under representative loads, common-mode and thermal measurements, and accepted-power field comparison against a controlled reference across the claimed bands.