Why Measuring a 49:1 EFHW Tells You It’s a Fantasy on 80–10 m
Hook a VNA to a 49:1 transformer and you’ll quickly see why the “multiband EFHW” promise is mostly marketing. What you’re really measuring is not wideband impedance transformation, but the tug-of-war between inductance, leakage, and parasitics.
What You Actually Measure
- At 80 m — The primary inductance is too small. The core draws heavy magnetizing current, runs warm, and burns power. Adding a shunt capacitor can flatten the analyzer trace, but it doesn’t remove the losses nor the power loss. The transformer is still inefficient.
- Above 14 MHz — Parasitic capacitance between windings dominates. The transformer flips capacitive and passes RF poorly, it acts more like a filter than a transformer. That’s why many “80–10 m EFHWs” look fine on 40/20 but unravel on 17/15/12/10. Adding a shunt capacitor can flatten the analyzer trace but that capacitor is essentially a band-extension crutch: it pulls the impedance trace closer to 50 Ω on higher HF, but adds loss and variability.
- Across the sweep — What you see are the limits of physics: a tiny two-turn primary cannot behave like a clean broadband device. The capacitor trick only moves the problem around.
Bottom line: A 49:1 does not scale across 80–10 m with good efficiency. Measuring one confirms it — inductive at the low end, capacitive at the top, lossy in between. The “multiband EFHW” is a convenient fantasy, not a clean solution.
Better Choices for Real Performance
If your main interest is 80 m, a 4:1 EFOC (off-center-fed with counterpoise) runs at a saner impedance and avoids the high-voltage stress of a 49:1. For the higher HF bands, purpose-built dual-band UNUNs (68:1, 70:1, or even 49:1) give reliable performance without pretending to cover “everything” in one shot.
Mini-FAQ
- Does a shunt capacitor fix 80 m? — No. It makes the match look nicer but doesn’t cure the core heating and leakage losses.
- Why does 49:1 look capacitive at high HF? — Because winding capacitance resonates with the inductance. Above that point, the transformer stops behaving like a transformer.
- So what works better? — For 80 m, an EFOC with a 4:1 UNUN. For 40/20 or dual-band coverage, properly wound high-ratio UNUNs. Just don’t expect 80–10 m from one 49:1.
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