Understanding Antenna Current Distribution and Impact on Performance
Updated August 21, 2025
Antenna efficiency is not just a matter of matching impedance or using a tuner. One of the most overlooked yet crucial factors is current distribution — particularly for horizontal antennas. Understanding how current behaves across the antenna structure, and how this interacts with ground losses and antenna height, reveals why some designs perform better than others, even if they seem electrically similar.
- EFHW: Feed at a voltage maximum, current minimum. The current maximum (where most radiation occurs) is near mid‑span. If that section is low over ground, losses rise.
- Near‑resonant OCF (EFOC): Feed nearer a current‑rich point. More current is elevated, improving radiation and reducing ground interaction.
- Practical takeaway: At modest heights (e.g., 8–10 m on 80 m), EFOC layouts typically place more current higher above ground than EFHWs.
Current Distribution and Radiation Efficiency
In any antenna, most of the radiation comes from where the current is strongest. For horizontal wire antennas, this is typically around the center for resonant dipoles and off‑center‑fed configurations. However, in end‑fed antennas — especially end‑fed halfwaves (EFHWs) — the current distribution is very different.
In an EFHW, the feed point is at a voltage maximum and a current minimum. This means very little current is present at the feed point, and the current maximum — where most radiation happens — is halfway down the wire. This is fine if the wire is installed high enough and in free space. But when installed close to the ground or over lossy soil, that current maximum sits low, and its interaction with the ground becomes inefficient.
Ground Losses and the Role of Height
Ground losses are resistive losses that occur due to the proximity of the antenna’s current‑carrying sections to the earth. These losses become more pronounced when the radiating current flows near the surface — especially at lower frequencies where the wavelength (lambda) is long.
For example, on 80 meters (λ ≈ 80 m), even 10 meters of height is only 1/8 wavelength. If the current maximum of an EFHW sits at 3–5 meters above ground, efficiency drops significantly due to interaction with the earth — not because of SWR, but because of actual radiated power loss.
By contrast, near‑resonant antennas like the EFOC29, EFOC17, and EFOC8 place the feed point somewhere along the wire where current is much higher — closer to a current maximum rather than a minimum. This results in better coupling to free space and less susceptibility to ground losses, especially when mounted at modest heights.
Why Near‑Resonant EFOC Antennas Outperform EFHWs at Realistic Heights
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Better Current at the Feed Point
EFOC designs are not high‑impedance voltage‑fed like EFHWs. They present moderate impedance and are fed at a current‑rich point. This improves radiation efficiency, especially when the feed point is near the shack and not suspended high above ground. -
Improved Matching
EFOC antennas typically use a 4:1 impedance transformer, not the high‑ratio 49:1 or 64:1 transformers needed for EFHWs. Lower ratios mean less transformer loss (especially at higher power) and less susceptibility to imbalance or coax common‑mode. -
Current Maximum at a Practical Height
Since the current maximum of an EFOC is not at the far end of the wire, but often somewhere between 1/3 and 2/3 along the span, it ends up being higher above ground in real‑world installations. This elevates the radiating current and reduces ground coupling. -
Balanced Performance Across Bands
While EFHWs are promoted as multiband antennas, they are most efficient on half‑wave (and some harmonic) bands. Off‑resonant operation is often lossy and pattern‑unstable. EFOC designs, when cut close to useful resonant multiples (e.g., 29 m for 40/20/15/10 m; 17 m for 20/17/10 m), show more stable SWR and pattern behavior across those bands.
Summary: Height, Lambda, and Real Efficiency
For horizontal antennas, height in wavelengths matters far more than physical length in meters. A 29‑meter EFOC installed at 8–10 meters height can easily outperform a 40‑meter EFHW at the same height due to better current distribution and lower transformer losses. Ground loss rises sharply as current flows near lossy earth. Designing antennas so that the radiating current is placed higher — and the feed point is not starved of current — pays off in real‑world signal strength and SNR.
EFHWs still have their place — especially in portable setups or where only a single support point is available — but for permanent stations, near‑resonant off‑center antennas like the EFOC series are more forgiving, efficient, and stable.
Mini-FAQ
- Where does most antenna radiation come from? — From sections with the highest current, not the highest voltage.
- Why are EFHWs inefficient at low heights? — Their current maximum sits low above ground, increasing resistive ground losses.
- Why do EFOCs use only a 4:1 transformer? — They are fed closer to a current maximum, so their feedpoint impedance is moderate, reducing transformer loss.
- Is height more important than exact length? — For horizontal antennas, yes. Elevating the current maximum is the primary efficiency driver.
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