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The Hidden Trap of EFHW Transformers: How "Magic Capacitors" Fix One Problem But Create Another

The Double-Edged Sword of Parallel Capacitors on EFHW Primaries

End-Fed Half-Wave (EFHW) antennas are beloved for their simplicity and versatility—but they’re also the most misunderstood antennas in amateur radio. One commonly overlooked topic is the use of parallel capacitors on the primary of the matching transformer. When used correctly, these capacitors can compensate for transformer losses on the lower bands. But if you’re chasing a perfect SWR curve without understanding what’s happening electrically, you’re walking into a trap.

The Intended Role: Compensating for Low-Frequency Losses

On the low bands (160m, 80m, sometimes 60m), the primary winding of the EFHW transformer suffers from increased copper losses and reduced coupling efficiency. To counteract this, a small value capacitor is placed in parallel with the primary winding. This introduces a resonant peak that lifts the transformer's efficiency on the lower bands, effectively reducing insertion loss and improving power transfer.

In short, this is smart RF engineering: you’re correcting a frequency-dependent weakness in the transformer.

The Side Effect: Creating a Resonant Notch or "Blind Spot"

However, adding this capacitor comes at a price.

As with any LC resonance, you’re introducing a frequency-specific behavior. While the capacitor boosts efficiency at one end of the spectrum, it also creates a blind spot—an impedance trap—usually between 20m and 10m. This manifests as a deceptively low SWR valley. Many operators see this as a feature. It’s not.

That 1:1 SWR in the blind spot doesn’t mean efficiency. In fact, your signal may be getting absorbed by the transformer or mis-routed due to poor radiation efficiency at that frequency. You’re seeing a good match, not good performance.

The Real Danger: Misusing the Notch

Some manufacturers and DIYers lean into this effect. They tune the capacitor to produce a convenient 1:1 SWR notch on a band like 15m or 12m. Then they advertise it as "multi-band capable." What you’re really getting is a resonant dip with poor radiation efficiency. The capacitor has become a marketing gimmick.

This is especially misleading for new operators, who naturally assume SWR = performance. In this case, it’s a trap.

The Fix: Use Capacitors for Compensation, Not Tuning

If you’re using a parallel capacitor, make sure it’s there to flatten losses at the low end—not to chase an idealized SWR curve across all bands. Use modeling tools, analyze your feedpoint impedance, and measure real-world performance—not just SWR.

And remember: in an EFHW, radiation happens along the wire—not in the transformer. If your capacitor is turning the transformer into a dummy load at some frequencies, you’re not radiating. You’re just wasting power.

Conclusion: Smart Use, Not Abuse

Parallel capacitors are a powerful tool—but only when used intentionally. Don’t let the convenience of a pretty SWR chart blind you to what’s really happening. Understand the physics, and use these capacitors to fix real problems, not to fake a performance curve.

Because in RF, what you don’t see can definitely hurt you.

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Written by Joeri Van DoorenON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.

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