The Hidden Trap of EFHW Transformers and “Magic Capacitors”
Updated August 21, 2025
End-Fed Half-Wave (EFHW) antennas are beloved for their simplicity and versatility — but also among the most misunderstood. A common example is the use of parallel capacitors on the transformer’s primary. If you’re chasing a perfect SWR curve without understanding the physics, you’re stepping into a trap.
The Problem Isn’t SWR — It’s Physics
On higher bands (17–10 m), an EFHW becomes a multi-halfwave radiator. Multiple current peaks appear, feedpoint impedance swings wildly, and transformer stress skyrockets. Small changes in wire length, height, or surroundings shift the impedance, making the design unpredictable and lossy.
The Myth of the Magic Capacitor
Some builders add a capacitor across the primary to “fix” higher-band behavior. Yes, it can create an SWR dip. But that dip rarely corresponds to efficient radiation. Instead, you’re resonating the transformer itself — often creating a lossy resonator or effective dummy load. The result: your SWR meter smiles while your signal weakens.
This trick is commonly used to fake “coverage” on 15 m or 12 m. In reality, little useful radiation occurs. What you’re matching is the transformer, not the antenna.
The Real Fix: Drop Inefficient Bands
At RF.Guru, we design EFHWs only for bands where current distribution supports true efficiency — typically 160–20 m. For higher bands, we recommend designs like our EFOC8, which is naturally efficient from 10–20 m without capacitors, traps, or hacks.
Conclusion: Respect the Antenna’s Limits
If you need a capacitor to make an EFHW “work” on a band, you’re forcing physics. It’s smarter to use the right antenna for the right range. Capacitors won’t create efficiency — they just make your SWR meter happy while your ERP suffers.
In RF, what you don’t see can definitely hurt you.
Mini-FAQ
- Does a capacitor make EFHWs more efficient? — No. It creates SWR dips by resonating the transformer, not by improving radiation.
- Why are EFHWs weak on higher bands? — Because they behave as multi-halfwave structures, with erratic impedance and lossy current distribution.
- What’s a better option for 10–20 m? — A purpose-designed EFOC, which runs efficiently across those bands without tuning tricks.
- Is a perfect SWR always good? — No. SWR dips can hide inefficiency if achieved through loss or reactive tricks at the transformer.
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