Understanding Antenna Current Distribution and Impact on Performance

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.

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 ≈ 80m), 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

  1. Better Current at the Feed Point
    The 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.

  2. Improved Matching
    EFOC antennas typically use a 4:1 impedance transformer, not the high-ratio 49:1 or 64:1 transformers needed for EFHWs. These lower-ratio transformers have significantly less loss — especially at higher power — and are less sensitive to imbalance or coax common-mode issues.

  3. 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 results in better radiation and reduced ground coupling.

  4. Balanced Performance Across Bands
    While EFHWs are promoted as multiband antennas, they only operate truly efficiently when used as full or halfwave antennas. Off-resonant harmonics are often lossy and unstable in pattern. EFOC designs, when cut close to resonant multiples (like 29m for 40/20/15/10m, 17m for 20/17/10m, etc.), show more stable SWR and better pattern control 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 increases exponentially as current flows near lossy earth. Designing antennas where the radiating current is placed higher — and where the feed point is not starved of current — pays off in real-world signal strength and performance.

EFHWs still have their place — especially in portable setups or where only a single support point is available — but in permanent installations, near-resonant off-center antennas like the EFOC series are far more forgiving, efficient, and stable.

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Written by Joeri Van Dooren, ON6URE – 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.