EFHW 20M-10M The Ground Sensitivity Problem
Updated: September 21, 2025
Why EFHW Antennas Get Harder on the Higher Bands
The end-fed half-wave (EFHW) is simple, popular, and effective on many HF bands. But not all bands treat it equally. While the EFHW can be very stable from 160–40 meters, many operators get frustrated with its instability on 20, 15, or 10 meters. Let’s explore why.
Lower Frequencies: More Forgiving
On 160 or 80 meters, an EFHW is long — up to 40–80 meters of wire. Because the wavelength is so large, small environmental changes (trees, gutters, soil moisture) are only a tiny fraction of the system. As a result, the impedance doesn’t shift much and SWR remains stable. Even heavy rain or someone walking under the wire usually won’t move resonance in a noticeable way.
Higher Frequencies: Sensitive and Shifty
On 20 meters and up, the opposite is true. A half-wave is now only 10 meters (or less). That shorter wavelength means:
- Stray capacitance is significant: A few pF of rain or a wall nearby can shift the feedpoint impedance by hundreds of ohms.
- Ground interaction grows in percentage: A couple of ohms of ground loss can represent a big fraction of the match.
- Environmental coupling dominates: Small objects, moisture, or coax routing changes can pull resonance around quickly.
This is why EFHWs that feel rock-solid on 80 m often “wander” on 20 m — every small change matters more as frequency rises.
Core Losses Grow with Frequency
The EFHW relies on a 49:1 or 64:1 transformer to tame the thousands of ohms at its end. While these work well on low bands, at higher frequencies their weaknesses show:
- Ferrite loss tangent rises: Core heating increases, reducing efficiency.
- Parasitics matter more: Leakage inductance and winding capacitance distort the match. The “magic capacitor” only hides this effect — it doesn’t fix it.
- Skin effect adds copper loss: Higher frequency means higher surface resistance in the windings.
- Relative losses increase: At 80 m, 50 Ω of core loss against ~3 kΩ radiation resistance is negligible (~1.6%). At 20 m, where transformed impedance is lower and bandwidth narrower, the same 50 Ω is a much bigger slice of efficiency.
This is why many hams notice their EFHW transformers staying cool on 80 m but warming up quickly on 20–10 m under the same power.
On 80 m, an EFHW transformer may see ~2.5–3 kΩ with moderate reactance, and small variations have little impact. On 20 m, the same system may see 2–3 kΩ ± several hundred ohms. That swing, plus rising ferrite loss, makes the antenna feel unstable and less efficient.
Practical Takeaway
EFHWs are excellent on the lower HF bands where the wavelength makes them forgiving and stable. On 20 m and above, expect them to be touchier, less efficient, and more prone to SWR shifts from environmental or weather changes. For high-band reliability, many operators prefer center-fed or off-center-fed solutions because they are more stable and more efficient.
Mini-FAQ
- Why is my EFHW stable on 80 m but not on 20 m? — Because small environmental changes are a larger fraction of the system at higher frequencies.
- Does this mean EFHWs are bad? — Not at all. They’re practical, especially on low bands. Just understand their limits on higher bands.
- Would an OCF be better on high bands? — Yes, an off-center-fed dipole has lower and more stable feedpoint impedance, making it less sensitive to small changes.
- Can chokes help stability? — They prevent common-mode currents, but they won’t stop the inherent frequency shift caused by environmental coupling.
Questions or experiences to share? Feel free to contact RF.Guru.
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