The Back-to-Back EFHW Transformer Measurement Myth

Updated November 11, 2025

Why “Back-to-Back” EFHW Transformer Tests Don’t Prove Efficiency

In amateur radio circles, back-to-back testing of End-Fed Half-Wave (EFHW) transformers has become a popular party trick: connect two identical transformers, measure power in and out, and declare the loss “known.” It looks neat, feels scientific, and is very often used to claim “this EFHW system is 98% efficient.”

Here is the core problem in one sentence: a back-to-back test only measures winding and core losses when both transformers see a perfectly matched, purely resistive 50 Ω load… while a real EFHW wire is a high-impedance, mostly reactive load at the feedpoint.

What People Actually Do in a Back-to-Back Test

Power test:

Radio → Wattmeter → EFHW transformer A → EFHW transformer B → Wattmeter → 50 Ω dummy load

VNA test:

VNA Port 1 → EFHW transformer A → EFHW transformer B → VNA Port 2

Then you often hear something like:

• “Input is 100 W, output is 94 W → 6 W lost in two transformers → 3 W each → about 0.13 dB loss per transformer. So the EFHW is almost lossless!”

It feels logical… but it’s drawing the wrong conclusion. That number tells you how the pair of transformers behaves under one very special condition: both transformers are working between 50 Ω ports, with a purely resistive load, in almost perfect match.

What the Back-to-Back Test Really Measures

In that setup, each transformer is essentially doing this job:

• See something close to its design impedance.
• Operate in a nearly pure resistive environment.
• Pass differential-mode power from one 50 Ω port to the other.

So what are you actually measuring?

• Winding copper loss under moderate current.
• Core loss under modest flux swing.
• Insertion loss (S₂₁) of a 50 Ω ↔ 50 Ω two-port, under ideal conditions.

You are not testing the transformer as used in an EFHW antenna. You are only testing how efficient it is as a nearly perfect 50 Ω transmission component.

EFHW Feedpoints: High Impedance and Mostly Reactive

A real “end-fed” half-wave wire does not present a nice 50 Ω resistor at the feedpoint. At or near resonance, a practical EFHW feedpoint:

• Often sits somewhere in the range of 2–5 kΩ of resistance,
• Plus a significant reactive component (jX) because of height, ground, surroundings, wire routing, and couplings.

Two important consequences:

• The transformer is almost never working into a purely resistive load.
• Most of the current at the primary is reactive current — it circulates in the transformer, heats the core and copper, but does not produce radiated power.

In other words, a real EFHW transformer spends its life handling a high-Z, reactive load under significant SWR, not a calm 50 Ω resistor. That operating point is exactly what the back-to-back test carefully avoids.

Resistance vs. Reactance: Why It Matters So Much

Many EFHW myths quietly ignore the difference between resistance and reactance. “Impedance is impedance, 2.5 kΩ is 2.5 kΩ” — but that’s not how power loss and heating work.

• Resistance (R) is where real power is dissipated as heat.
• Reactance (X) stores and releases energy each RF cycle, causing circulating current that can be large even when the real power is modest.

In a high-Q, mostly reactive situation (like many EFHW feedpoints), the transformer primary can see large currents for very little radiated power. Those circulating reactive currents:

• Increase core flux swing.
• Increase copper I²R loss.
• Push the core closer to saturation or overheating.

Back-to-back tests remove this reactive stress on purpose. That’s why they look so “good.”

The Coax Shield: The Invisible Third Conductor

A real EFHW system is not just a neat two-wire system. You usually have at least three conductors participating:

• Coax inner conductor (forward current).
• Inner surface of the shield (return for differential current).
• Outer surface of the shield and “ground” stuff (common-mode path: mast, feedline, shack, rain gutters, you).

In practice, this means:

• The coax shield often becomes part of the radiator.
• Pattern, noise pickup, and RF-in-the-shack are strongly affected by common-mode current.
• The transformer design and choking strategy control how bad this becomes.

In a back-to-back test, the setup is highly symmetrical and both ends are neatly terminated. There is almost nothing to drive common-mode current — the coax shield is artificially “quiet.” So one more crucial part of real EFHW behavior simply disappears from the measurement.

“I Ran 500 W Through It and It Stayed Cool” – Still Not Proof

Even a high-power back-to-back test only proves that the transformer stack is happy when it sees something close to 50 Ω on each side. Under those conditions:

• Flux density in the core is moderate.
• Reflected reactive power is low.
• Standing wave ratios are small.

In real EFHW operation, especially at band edges or on bands where the wire length is “ugly”:

• SWR can easily be 3:1–5:1 or worse.
• Voltages and currents inside the transformer climb sharply.
• Core heating and voltage stress can be much higher than in the bench test.

So “500 W back-to-back without smoke” only shows that the transformer can survive easy-mode operation. It does not certify efficiency or safety in the real EFHW antenna system.

The 100–120 pF “Magic Capacitor” Across the Primary

A common tweak is to add a 100–120 pF capacitor across the primary of the EFHW transformer. On the bench, especially in a back-to-back fixture, that often:

• Flattens the SWR curve into 50 Ω.
• Makes S₂₁ look slightly smoother.
• Gives an overall “nicer” plot in the mid/high HF range.

That can be useful in a real antenna design, where the capacitor is part of a deliberate matching and compensation strategy. But in the back-to-back test it mainly does this:

• Retunes the 50 Ω-to-50 Ω test network so that the transformers look prettier to the VNA.
• Further optimizes an already idealized condition (purely resistive, low SWR).
• Hides issues you would see under reactive, high-Z loads instead of revealing them.

So yes, 100–120 pF changes the measurement. But it only “improves” the response of an already unrealistic test. It does not suddenly make the back-to-back method representative of a real EFHW antenna.

Z_primary,total = ( 1 / (j ω L_p) + j ω C_p + 1 / R_loss )⁻¹

How to Evaluate an EFHW Transformer in a Way That Actually Matters

1. Use realistic or synthetic “ugly” loads, not only 50 Ω

• Use a real EFHW wire, or build a synthetic test load that looks like a few kΩ with reactance (R + jX).
• Sweep with a VNA and look at S₁₁ and S₂₁ into that complex load.

2. Drive it at power into that realistic load

• Run your usual duty cycle (SSB, FT8, RTTY…) into the transformer and measure core temperature over time.
• Watch for SWR drift with heating — that hints at changing core properties and increasing loss.

3. Measure common-mode current on the coax shield

• Use a clamp-on RF current probe near the feedpoint and a few meters down the coax.
• Compare different transformer designs and choking strategies — look for reduced shield current, not just “nice” SWR.

4. Test in the actual antenna installation

• Install the transformer where you intend to use it.
• Check on-air reports, SNR on receive, noise pickup, RF in the shack, and long-term thermal behavior.

Technical Deep Dive: Why Back-to-Back Cancels the Hard Part

Think of one EFHW transformer (for example a 49:1 unun) as:

• An ideal transformer with turns ratio n (≈7:1 for 49:1 impedance ratio), plus
• Leakage inductance, magnetizing inductance, copper resistance, core loss, and stray capacitances.

In the back-to-back setup:

50 Ω source → transformer A → transformer B → 50 Ω load

Transformer B is (approximately) the inverse of A. The overall chain becomes:

• A 50 Ω-in, 50 Ω-out network with high return loss.
• Very low insertion loss under matched, differential-mode conditions.
• A configuration where most of the impedance conversion role cancels out.

Your wattmeter or VNA is therefore measuring the easiest possible job for the transformer: modest voltage, modest current, small reactive component, minimal reflections.

Contrast that with a real EFHW feedpoint:

• High resistance (kΩ range) and typically large reactance.
• Significant reflection coefficient Γ and SWR at many frequencies.
• Strongly frequency-dependent behavior that stresses the core and windings most where you care about it least on the bench.

That is why back-to-back testing is fundamentally a best-case loss measurement on the winding and core — not a realistic efficiency measurement for an EFHW antenna system.

Conclusion: Back-to-Back Is a Comforting Crutch, Not a Real Test

Back-to-back EFHW transformer tests:

• Measure winding and core loss only under ideal, purely resistive 50 Ω conditions.
• Hide the high-impedance, mostly reactive nature of real EFHW feedpoints.
• Ignore common-mode currents and coax shield behavior.
• Tell you nothing about voltage breakdown, saturation margin, or heating under ugly SWR.

If you want to know how good your EFHW transformer really is, you must look at it where it actually works: in front of a high-Z, reactive antenna, with real reflections, common-mode currents, and real power. The neat back-to-back number is comfortable… but it is not the truth about EFHW efficiency.

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