The Back-to-Back EFHW Transformer Measurement Myth
Updated August 21, 2025
In amateur radio, one of the most misleading practices is back-to-back testing of End-Fed Half-Wave (EFHW) transformers. It sounds tempting: connect two identical transformers, measure power in/out, and subtract to find “loss.” Simple? Logical? Unfortunately, wrong. Here’s why.
The Measurement Setup
A typical setup looks like:
TX → Wattmeter → Transformer A → Transformer B → Wattmeter → Dummy LoadOr in hobby VNA use:
VNA Port 1 → Transformer A → Transformer B → VNA Port 2At first glance, this seems to measure efficiency. In reality, you’re measuring how well one transformer can undo the other — under assumptions that the load is purely resistive, frequency response is linear, and the system is matched. None are true in live antennas.
The Real Load Isn’t a Dummy Load
EFHW and similar end-fed antennas present a complex impedance (R, L, C) that varies with frequency, height, and layout. A 50 Ω dummy load or simple S21 test tells you nothing about how the transformer performs under real antenna stress.
No Reflection = No Insight
Back-to-back tests minimize reflections. Real EFHW systems, however, often see SWR of 5:1 or higher. That reactive stress pushes cores and insulation to their limits. The tidy bench test ignores this completely.
Coax Shield Return Currents Are Missing
In reality, undefined returns cause differential currents to flow on the coax shield. They radiate, distort the pattern, and raise noise. In a back-to-back test? Perfect symmetry keeps the shield clean — something that never happens on an actual end-fed install.
High-Power Tests Still Miss Reality
Running 500 W into back-to-back transformers only shows differential transfer efficiency. You still miss:
- Voltage breakdown under mismatch
- Reactive loss under real SWR
- Coax shield radiation
A Better Approach
- Test against complex loads, not dummy resistors
- Drive into real EFHWs, OCFs, or wires and monitor heat
- Measure shield return currents with RF current probes
- Do wideband sweeps with reactive terminations
Most importantly: validate the transformer as part of the whole antenna system, not isolated in a lab shortcut.
Conclusion: Back-to-Back Is a Crutch
Back-to-back testing provides the illusion of data while ignoring every non-ideality that matters: mismatches, reflections, saturation, and shield currents. If you want meaningful results, measure under real antenna conditions — not tidy lab myths.
The real antenna system is messy. Embrace the mess — or be fooled by neat but useless tests.
Mini-FAQ
- Why is back-to-back EFHW testing flawed? — It uses dummy loads and symmetry, hiding the reactive, lossy reality of antennas.
- Does back-to-back show core heating? — Only partially, and not under realistic mismatched stress.
- What’s missing in such tests? — Coax return currents, mismatch loss, voltage breakdown, and reactive effects.
- What’s the better method? — Measure into complex loads or real antennas, observe heating, and monitor shield currents.
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