Feed Point Impedance vs. Feedpoint Height Above Ground for End-Feds
Updated August 21, 2025
End-fed antennas are popular for their simplicity, but their feed point impedance varies strongly with feedpoint height above ground, orientation, and surrounding objects. This directly affects matching, efficiency, and pattern stability. Below we compare three families: the End‑Fed Half‑Wave (EFHW) with 49:1, the End‑Fed Off‑Center (EFOC) with 4:1, and the End‑Fed Random Wire (EFRW) with 9:1, with practical guidance by height.
- EFHW (49:1): Very high impedance (≈2–5 kΩ). Height has a large effect; performs best when the wire spans near ½λ and the current maximum is not too close to lossy ground.
- EFOC (4:1): Moderate impedance (≈150–600 Ω). More tolerant of modest heights; current‑rich feed improves real‑world efficiency.
- EFRW (9:1): Highly variable impedance; always plan on a tuner. A defined counterpoise stabilizes behavior and reduces losses.
End‑Fed Half‑Wave (EFHW) with a 49:1 Transformer
An EFHW is cut near ½λ on its lowest band, presenting ~2000–5000 Ω at the end. A 49:1 autotransformer brings this into the 30–120 Ω region for coax.
Effect of Feedpoint Height Above Ground
- Low heights (< 10 m / < 33 ft): Apparent impedance often drops due to strong ground coupling and loss; efficiency and pattern suffer.
- Intermediate heights (10–15 m / 33–49 ft): Impedance typically stabilizes in the ~2–4 kΩ region; efficiency improves as the current maximum lifts.
- Higher elevations (> 15 m / > 49 ft): More stable impedance with smaller swings from local environment.
- Geometry: A sloper often reads slightly lower Z than a horizontal span; near‑vertical runs show more variability from nearby objects and ground.
End‑Fed Off‑Center (EFOC) with a 4:1 Transformer
When the wire length is chosen near useful resonances (½λ, 3/2λ, 5/2λ), the feed point is closer to a current‑rich position, yielding ~150–600 Ω. A 4:1 current unun usually lands near 50 Ω with good common‑mode behavior.
Effect of Feedpoint Height Above Ground
- Low heights (< 5 m / < 16 ft): Impedance tends to read lower than expected from ground capacitance; efficiency is workable but not ideal.
- Practical heights (5–10 m / 16–33 ft): Z stabilizes in the target window (~150–500 Ω), making 4:1 matching effective with modest coax sensitivity.
- Higher (> 15 m / > 49 ft): Stable performance with minor frequency‑dependent variations.
End‑Fed Random Wire (EFRW) with a 9:1 Transformer
A random‑length wire into a 9:1 unun is inherently variable in Z across frequency and height and requires an ATU. A defined counterpoise reduces common‑mode and stabilizes impedance.
Effect of Feedpoint Height Above Ground
- Very low (< 2 m / < 6.5 ft): Strong ground effects drive wide swings; Z may drop into ~100–1000 Ω.
- Low‑to‑moderate (2–5 m / 6.5–16 ft): Z commonly ranges ~500–5000 Ω, highly dependent on length, bends, and surroundings.
- Higher (> 10 m / > 33 ft): Variations smooth out somewhat, but remain unpredictable over wide frequency coverage.
- Counterpoise: Essential for repeatability; reduces ground losses and stabilizes tuner behavior.
Summary Table — Feed Point Impedance vs Feedpoint Height
(Indicative ranges; installation, soil, nearby objects, wire routing, and band selection can shift values.)
| Feedpoint Height | EFHW (49:1) | EFRW (9:1) | EFOC (4:1) |
|---|---|---|---|
| 1–2 m (3–7 ft) | — | ~100–1000 Ω | — |
| 2–5 m (7–16 ft) | — | ~500–5000 Ω | ~100–300 Ω |
| 5–10 m (16–33 ft) | — | — | ~150–500 Ω |
| 10–15 m (33–49 ft) | ~2000–4000 Ω | — | ~150–500 Ω |
| 15+ m (49+ ft) | ~3000–5000 Ω | ~1000–6000 Ω | ~200–600 Ω |
Conclusion
Feedpoint height strongly influences end‑fed impedance and real efficiency. As a rule of thumb: EFHW (49:1) behaves best when the span is high enough to lift the current maximum away from lossy ground (≈≥10 m / 33 ft on lower bands). The EFOC (4:1) is robust at ~5–10 m / 16–33 ft, thanks to a current‑richer feed and moderate impedance. The EFRW (9:1) is flexible but variable; always plan for a counterpoise and tuner.
Optimizing height and orientation — not just transformer ratio — yields more consistent matching, better SNR, and stronger field reports.
EFHW vs EFOC — Two Voltage‑Driven Antennas, Two Smart Solutions
The End‑Fed Half‑Wave Myth — Why Most EFHWs Are Doing It Wrong
Mini-FAQ
- Why does height change feedpoint impedance? — Ground capacitance and image currents modify the wire’s current/voltage distribution and thus the observed impedance.
- Do I always need a counterpoise? — For EFRW and many EFHW installs, yes. A defined return path stabilizes impedance and reduces common‑mode.
- Is 49:1 always right for EFHW? — Not always. Real feedpoint Z can deviate widely with height and geometry; some cases prefer ~64:1 or different lengths.
- Why is EFOC easier to match? — It’s fed nearer a current maximum, so the impedance is moderate and transformer losses are lower.
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