EFHW: High Feedpoint Resistance Is Not Free Efficiency

Why 2,450 Ω does not make an end-fed half-wave intrinsically superior.

An end-fed half-wave can be an excellent antenna. It offers a convenient support arrangement, can be light enough for portable work and can radiate efficiently when the wire, matching network, return path and installation are engineered well.

None of that follows from the number 2,450 Ω.

Greg Mihran’s July 2026 antenna primer argues that the radiation resistance of a vertical rises from about 37 Ω at a quarter wavelength to about 2,450 Ω near 0.47 wavelength, calls the latter “good resistance”, adds an assumed 12 Ω counterpoise loss and calculates 99.5% efficiency. The slide then uses that arithmetic to suggest that a half-wave vertical is inherently more efficient than a quarter-wave vertical requiring over 100 surface radials.

The calculation is tidy. The physical inference is not.

Give the EFHW credit for what it really does

The EFHW is not pseudo-science. It is a legitimate way to excite a half-wave mode near a high-voltage, low-current point. A suitable network transforms the high terminal impedance to something useful for a 50 Ω station. A deliberate counterpoise or controlled section of feedline can provide the other RF terminal; an isolated matching network can help keep that return current out of unintended conductors.

That can be mechanically attractive. One end of the wire is close to the equipment, the far end can be raised with a single support and harmonic operation can make multiband designs possible. A careful design can perform very well.

The error begins when convenience is converted into an intrinsic efficiency advantage and the high input resistance is presented as its cause. Feedpoint resistance is not a reservoir of useful watts. It is a ratio measured or calculated at one electrical location.

What the primer actually says

The central contradiction is already visible on slide 81. If the same half-wave can be described by roughly 70 Ω at its centre and several thousand ohms at its end, resistance magnitude cannot itself be the source of efficiency. The feedpoint moved. The laws of conservation of energy did not.

2,450 Ω is a feed location, not a performance bonus

A resonant half-wave has a standing current distribution. Current is high near the centre and low near an open end; voltage follows the opposite trend. Input impedance is the ratio of terminal voltage to terminal current:

Moving the feed towards a current minimum makes the numerical input resistance large. It does not make the electromagnetic field contain more energy for the same accepted power. It changes the voltage and current needed to deliver those watts.

The AA5TB material cited by the primer explains exactly this current-and-voltage relationship and gives a practical end impedance range of 1,800–5,000 Ω. It also makes clear that the value depends on counterpoise length and configuration. Its later ground-loss discussion is introduced as the author’s thoughts “right or wrong”, not as a universal efficiency measurement.

Real EFHW input impedance varies with wire diameter, electrical length, height, slope, bends, harmonics, soil, nearby conductors, the return conductor, feedline routing and choke position. It may contain reactance. The number 2,450 Ω is a useful nominal design target for a 7:1 turns ratio; it is not a material constant belonging to every EFHW.

Radiation resistance needs a named reference current

Radiation resistance is an equivalent resistance defined so that a chosen reference current accounts for the radiated power. With RMS current:

Move the reference to a point where current is ten times smaller and both equivalent resistances become 100 times larger for the same radiated and dissipated powers. Their ratio—and therefore efficiency—does not change.

This is the missing qualifier in the phrase “high radiation resistance promotes high efficiency”. A larger ratio of radiation resistance to total correctly referred resistance indicates higher efficiency. A larger radiation-resistance number created by choosing a lower reference current does not.

Rudy Severns, N6LF, states the reference explicitly in Radiation and Ground Loss Resistances, Part 1: radiated power is related to radiation resistance referenced to the feedpoint current, and radiation resistance cannot be measured directly. In Part 2, he demonstrates that even the familiar quarter-wave value changes with soil and radial-system design before converging towards the ideal value as the ground system improves.

The 12 Ω question: referred to which current?

Slide 85 assumes a “modeled loss of ~12 Ω in the counterpoise” and inserts it directly beside 2,450 Ω. That step might be valid in a fully disclosed equivalent circuit, but only after several conditions have been met:

• The 2,450 Ω must be the radiation component of the real part of the input impedance for the same lossy model, not merely the total resistance of a different perfect-ground model.
• The 12 Ω must represent dissipated counterpoise and ground power divided by the square of the same source current.
• Conductor, connection, transformer, choke, feedline and unintended common-mode losses must either be included or explicitly excluded.
• The complete geometry, material properties, ground model, source and power budget must be available for reproduction.

The primer supplies none of that derivation beside the number. It publishes an assumed loss and a quotient. That is not enough to establish that a physical 12 Ω has been referred correctly to a 2,450 Ω port.

A physical resistance read from a wire property, ground estimate or another antenna cannot simply be placed in series with a port-referred radiation resistance. The current through the loss matters. Distributed soil loss is an integral of fields and conductivity; conductor heating depends on current along the wire; transformer loss depends on flux, winding current, frequency and load. Each must first be converted to power, then referred consistently if an equivalent-resistance diagram is used.

The formula is not the fallacy. Mixing boundaries and current references inside the formula is.

The ideal-transformer test exposes the mistake

Imagine a 50 Ω antenna-port equivalent containing 40 Ω of radiation resistance and 10 Ω of loss resistance. Its radiation efficiency is 80%.

Now place an ideal lossless transformer in front of it with a 1:7 turns ratio. The impedance ratio is 49:1:

The transformer has created a 2,450 Ω port. It has not created efficiency. Radiation and loss resistances transformed together.

This is a thought experiment, not a claim that moving a feedpoint is identical in every respect to inserting a transformer. Its purpose is to test the alleged causal rule. If impedance magnitude alone created efficiency, a lossless transformer could manufacture efficiency merely by changing turns ratio. It cannot.

The real transformer is where the high impedance sends the bill

An EFHW’s large impedance does not eliminate a matching problem; it creates one. The network must support a large transformation ratio across the intended frequencies and power while controlling:

• core loss and heating;
• copper resistance and proximity effect;
• leakage inductance;
• inter-winding and stray capacitance;
• high RF voltage and dielectric stress;
• load-dependent mismatch and bandwidth;
• common-mode current through unintended capacitance and external conductors.

A nominal 49:1 marking specifies an impedance ratio under an idealised assumption. It does not specify insertion loss, power handling or behaviour into the installed antenna’s complex impedance.

The primer’s own component table makes this point. Slide 91 assigns the two Dominator transformer-and-choke combinations losses of 1.08 dB and 0.51 dB. Those correspond to power efficiencies of about 78.0% and 88.9%, respectively. If those figures are compatible with the separate 99.5% structural model and are multiplied, the resulting antenna-system efficiencies are approximately:

Those are conditional bookkeeping results, not new measurements; the primer does not publish enough test detail to establish that every number shares frequency, load, power and reference plane. They nevertheless show why “99.5% structural efficiency” cannot be presented as complete installed efficiency. The primer’s own feed-network loss is much larger than the half-percentage-point structural loss highlighted on slide 85.

Structural efficiency is not system efficiency

A useful loss budget keeps each boundary visible:

The denominator must accompany the percentage. A structure can be 99.5% efficient in a model while the transformer-input system is below 80%. Both figures can be mathematically consistent because they describe different boundaries.

There is always a second RF terminal

End feeding does not repeal Kirchhoff’s current law. The source has two terminals. If one terminal drives the half-wave wire, the return current must occupy something else: a deliberate counterpoise, a transformer secondary, the outside of the coax, a mast, station wiring, parasitic capacitance to the environment or some combination.

Slide 84 correctly recommends a counterpoise and choke. The overstatement is that a counterpoise automatically “finishes the half wavelength” and delivers “full radiation efficiency”. Return-path geometry is part of the antenna. It can change input impedance, current distribution, common-mode current, pattern and loss. A choke changes that network again; its location and common-mode impedance matter.

AA5TB’s own experiments are useful here. With an isolated coupler, removing the short return conductor prevented a match; other arrangements used the feedline or stray capacitance as the return. That supports the modest engineering statement that a high-impedance end feed may require only a small return current under carefully adjusted conditions. It does not prove that every short counterpoise has a universal 12 Ω loss or that every installation is 99.5% efficient.

Low SWR is not the missing efficiency measurement

The Dominator model plots use an SWR reference of 2,450 Ω. That is a statement about how closely the model input approaches its chosen high-impedance target. The later RigExpert images show low SWR at the 50 Ω side of the installed transformer. They demonstrate that the complete input can be matched on the displayed bands.

Neither result tells us how accepted power divides between radiation and heat. A lossy matching network often improves apparent bandwidth and can still show excellent SWR. A 50 Ω dummy load is the limiting example: perfect match, essentially zero useful antenna efficiency.

The field images therefore support “the antenna could be matched”. They do not validate the modelled 99.5%, +0.67 dBi gain or radiation angle. Those require a traceable power budget and comparative field or range measurements.

Does end feeding ever reduce a real loss?

Yes, in a specific design it can. Low current at the end may reduce heating in a particular connection or return-path element. A half-wave vertical can also move its strongest near-field interaction away from the base compared with a short, ground-fed quarter-wave. Geometry and current distribution can produce genuine performance differences.

But each benefit must be demonstrated through the actual current and power loss. The high-voltage end simultaneously increases electric-field stress and places demanding requirements on the transformer and insulation. Another feed arrangement may avoid the high-ratio transformer altogether. There is no universal winner independent of installation.

That is why “intrinsically superior” is the wrong conclusion. A fair comparison holds frequency, radiator material, orientation, height, environment and accepted power constant, then includes every matching and return-path loss. It compares radiation patterns as well as peak gain. It does not compare one antenna’s ideal structure with another antenna’s incomplete ground system.

What would prove the EFHW performance claim?

A reproducible 99.5% or superiority claim needs more than a resistance quotient and SWR screenshots. Publish:

1. The complete model. Source file, NEC engine, ground formulation, soil data, conductor properties, segmentation, loads, source position, counterpoise, mast, tripod, feedline and choke representation.
2. The model power budget. Accepted power, radiated power, wire loss, network loss and ground loss, with the efficiency definition and reference plane.
3. The complex source current. Enough current magnitude and phase data to show how the 2,450 Ω and the equivalent loss resistances were referred.
4. Transformer loss under the real load. Frequency, complex load, fixture, calibration, de-embedding, drive level, duty cycle, temperature and uncertainty.
5. Return-path evidence. Counterpoise current and common-mode current on the coax, mast and other conductors, before and after the choke.
6. A calibrated comparison. Rapid A/B field-strength or gain measurements against a known reference antenna, normalised to equal accepted power and repeated across relevant azimuths and days.
7. A complete system result. Combine structural, transformer, choke, feedline and mismatch terms without silently changing the denominator.

Contacts, signal reports and a low SWR are legitimate evidence that an antenna is useful. They are not laboratory measurements of radiation efficiency.

Takeaways you can trust

• An EFHW can be efficient and effective.
• Its high feedpoint resistance comes from feeding near a current minimum and voltage maximum.
• A high resistance number does not create radiated power.
• Radiation and loss resistances are meaningful only with a stated reference current.
• Every resistance used in an efficiency quotient must be referred to the same port and current.
• The nominal 2,450 Ω varies with the antenna, return path and environment.
• A 49:1 transformer transforms impedance; it does not add efficiency.
• Real transformer, choke and feedline losses reduce complete-system efficiency.
• A counterpoise, coax exterior or other conductor always provides the second RF path.
• Low SWR proves a match at the measurement plane, not radiation efficiency.
• Compare complete antennas at equal accepted power, not isolated resistance numbers.

In Summary

The primer starts with a useful model result: near a current minimum, a resonant end-fed half-wave can present an input resistance of a few thousand ohms. It then turns that descriptive number into a cause of efficiency.

That is the fallacy.

The same radiated and dissipated powers can be represented by very different equivalent resistances when the reference current changes. A centre-fed half-wave may be around 70 Ω; an end-fed version may be in the thousands. Neither number is an efficiency certificate. The decisive quantity is the ratio of radiated power to accepted power after the complete structure, return path and matching network have been counted.

The EFHW deserves to be judged as a complete antenna system. Done well, it may be an excellent choice. But 2,450 Ω is an engineering challenge to be matched and measured—not free efficiency.

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