Feed Point Impedance vs. Feedpoint Height Above Ground for End-Feds
End‑fed antennas are popular because they’re fast to deploy, but their feedpoint impedance is strongly affected by band (160–10 m), feedpoint height in wavelengths, geometry (flattop, sloper, inverted‑V, inverted‑L), and especially the return path (counterpoise + common‑mode on coax).
One important correction to many internet “rules”: feedpoint height is not a fixed requirement, and it has much more to do with impedance stability and return‑path behavior than with “does the antenna radiate”. Radiation comes from the current‑rich parts of the wire (current maxima), which may be high or low depending on shape — not just where the transformer sits.
- Think in wavelengths, not meters: the same 10 m feed end is ~0.06λ on 160 m but ~1.0λ on 10 m. So “needs to be high” can’t be a fixed number.
- EFHW (49:1 / 68:1 / 70:1): most efficient and predictable when used as monoband or octave dual‑band (harmonic) designs. “Multi‑band EFHW 160–10” operation is often possible, but rarely the most efficient.
- EFOC (4:1): a more current‑rich feed (lower feed voltage) is typically more forgiving at modest heights and in noisy real‑world installs.
- EFRW (9:1): inherently variable across 160–10 m; always plan on an ATU. A defined counterpoise + good choking is essential for repeatability.
- Most “height changed my SWR” stories are return‑path stories: if the coax isn’t well‑choked or the counterpoise isn’t defined, the feedline becomes part of the antenna and everything moves (SWR, pattern, noise).
- Inverted‑L EFHW: 160/80 (68:1), 80/40 (70:1), 40 m monoband (70:1)
- Flattop / Sloper EFHW: 40/20 (49:1), 20 m monoband (49:1)
Reason: efficiency (system efficiency, not just “it tunes”). EFHWs are at their best when they operate at (or very near) resonance on the intended bands. Octave dual‑band EFHWs (e.g., 160/80, 80/40, 40/20) are harmonic designs that keep the end as a voltage maximum on both bands — which is why they are typically the most efficient and repeatable EFHWs.
What “Feedpoint Height” Really Means (and Why It Isn’t a Fixed Rule)
Three different “heights” get mixed up:
- Feed end height: where the transformer sits. This mostly affects impedance stability, resonance shift, and common‑mode behavior because it’s a high‑voltage point.
- Average wire height: influences the radiation pattern (takeoff angle, nulls/lobes), especially on higher bands.
- Height of current maxima: the current‑rich parts do most of the radiating. This is often the “efficiency” height that matters.
This is why a low feedpoint (even near ground) can still perform very well — especially with an inverted‑L on low bands — as long as the return path is controlled (counterpoise + choke) and the current‑rich sections are positioned well.
Electrical Height Reality Check (160–10 m)
Same physical height ≠ same electrical height. This is the root of “it changed when I changed bands”.
| Band | 5 m feed end ≈ | 10 m feed end ≈ | What you’ll notice |
|---|---|---|---|
| 160 m | 0.03 λ | 0.06 λ | Electrically very low: end coupling to ground/objects strongly shifts resonance and measured impedance. |
| 80 m | 0.06 λ | 0.12 λ | Still low: SWR changes easily with environment and return‑path setup. |
| 60 m | 0.08 λ | 0.17 λ | Intermediate: reactive behavior depends heavily on routing and nearby objects. |
| 40 m | 0.13 λ | 0.25 λ | Moderate: impedance changes are often smaller, but pattern evolves with height/geometry. |
| 30 m | 0.17 λ | 0.33 λ | Often “non‑ideal” for EFHW lengths aimed at other bands; matching dominates. |
| 20 m | 0.25 λ | 0.50 λ | Higher electrical height; long‑wire patterns begin if the radiator is long in λ. |
| 17 m | 0.29 λ | 0.59 λ | Pattern becomes more directional; deep nulls appear depending on geometry/orientation. |
| 15 m | 0.33 λ | 0.67 λ | Small geometry changes can shift lobes/nulls dramatically. |
| 12 m | 0.42 λ | 0.83 λ | Very directional long‑wire behavior is common. |
| 10 m | 0.50 λ | 1.00 λ | Extremely geometry‑dependent: pattern and feedline coupling can dominate your results. |
End‑Fed Half‑Wave (EFHW) — 49:1 / 68:1 / 70:1 (and why dual‑band matters)
An EFHW is designed so the wire is near ½λ on its primary band. The end of a half‑wave is a high‑voltage / low‑current point, so the feedpoint impedance at resonance is typically in the kΩ range (and it often includes a reactive component that changes with installation).
Why EFHWs Are Usually Best as Monoband or Octave Dual‑Band
The most “efficient EFHW” use cases share a simple property: on the intended bands, the wire is operating at a natural standing‑wave condition where the end remains a voltage maximum and the impedance stays within a matchable region without extreme reactive stress.
- Octave dual‑band EFHWs (160/80, 80/40, 40/20) work well because the higher band is the 2nd harmonic of the lower band: the wire is roughly ½λ on the low band and roughly 1λ on the high band. The end is still a voltage maximum on both.
- “Everything from 160–10 m” EFHW operation typically forces operation on bands where the wire is not near a friendly resonance. You can often make it tune, but system losses usually increase: more reactive stress, higher voltages, more common‑mode risk, and more coax/ATU loss.
When we say “efficiency” here, we mean system efficiency: power at the transmitter vs. usable radiated field — including transformer/core losses, ATU losses, coax loss under mismatch, and unintended feedline radiation.
Transformer Ratios (Impedance Mapping)
These ratios don’t make an EFHW “right”; they decide what feedpoint impedance range lands near 50 Ω. As a simple mapping to ~50 Ω on the coax side:
| Ratio | End impedance that maps to ~50 Ω | Practical note |
|---|---|---|
| 49:1 | ≈ 2450 Ω | Often a strong fit for many 40/20 and 20 m EFHW installs (geometry and environment decide the final number). |
| 68:1 | ≈ 3400 Ω | Commonly useful when the real end‑impedance trends higher (typical in some low‑band/inverted‑L deployments). |
| 70:1 | ≈ 3500 Ω | Similar to 68:1; a small step that can help when the measured end‑Z is consistently higher on the primary band. |
| 64:1 | ≈ 3200 Ω | Another common ratio in the hobby; useful when measured end‑Z sits around this region. (Included here for completeness.) |
RF.Guru EFHW Designs (Geometry + Bands + Ratio)
| Design family | Bands | Geometry | Transformer ratio | Why it’s in the line |
|---|---|---|---|---|
| EFHW Dual‑Band | 160/80 | Inverted‑L | 68:1 | Octave harmonic pair; efficient low‑band solution when space/height is limited, with controlled return path. |
| EFHW Dual‑Band | 80/40 | Inverted‑L | 70:1 | Octave harmonic pair; strong real‑world efficiency and repeatability. |
| EFHW Monoband | 40 | Inverted‑L | 70:1 | When you want maximum 40 m performance without “multiband compromises”. |
| EFHW Dual‑Band | 40/20 | Flattop / Sloper | 49:1 | Octave harmonic pair; excellent efficiency for typical portable/home supports. |
| EFHW Monoband | 20 | Flattop / Sloper | 49:1 | High‑performance 20 m without the pattern/impedance compromises of forcing extra bands. |
Feedpoint Height — Correct Way to Think About It (EFHW)
- Height is not a fixed “must be high” number. It changes the end coupling (capacitance) and therefore shifts the measured R+jX.
- On 160/80: almost every practical feedpoint height is electrically low, so the end is strongly influenced by ground/objects. Expect resonance/impedance to move unless the return path is controlled.
- On 40/20: the same physical height is a larger fraction of a wavelength, so end coupling effects often look smaller — but pattern changes and lobes/nulls start to matter more.
- Low feedpoint ≠ low efficiency. Especially for inverted‑L low‑band systems, a near‑ground feedpoint can be excellent when the current‑rich parts of the antenna are positioned well and common‑mode is suppressed.
Geometry Matters (Flattop vs Sloper vs Inverted‑L)
- Flattop: predictable, clean geometry. Often easy to make repeatable once trimmed at the install height.
- Sloper: often a very effective compromise, but the feed end is frequently closer to objects/ground → more impedance drift if return path is not defined.
- Inverted‑L: excellent for low bands in limited space. It increases the importance of counterpoise + choking, because vertical sections and feedlines easily participate if you let them.
Why 17/15/12/10 m Are Usually Not “Ideal” on a Low‑Band EFHW
You can often tune an EFHW on 17–10 m, but it’s typically not the most efficient or predictable use of the system:
- Impedance is often far from the transformer’s sweet spot → more reactive stress and often more tuner dependence.
- Pattern becomes multi‑lobed with deep nulls → you can be very strong in one direction and very weak in another (direction lottery).
- Common‑mode risk rises on bands where the system is not operating near its intended standing‑wave condition.
End‑Fed Off‑Center (EFOC) — 4:1
EFOC designs move the feedpoint away from the extreme end so the feed becomes more current‑rich (lower feed voltage). That typically makes them more forgiving in real installs, especially when height is modest and the environment is “messy”. A 4:1 transformer often places typical working impedances closer to what coax and tuners like.
Height and Band Dependence (EFOC)
- Still affected by electrical height and geometry, but usually less twitchy than a true end‑fed voltage maximum point.
- On lower bands, the more current‑rich feed often translates into better practical efficiency (less transformer stress, fewer common‑mode surprises).
End‑Fed Random Wire (EFRW) — 9:1
A random wire into a 9:1 transformer is a wide‑coverage concept. Across 160–10 m the impedance can land almost anywhere, so an ATU is assumed. For repeatability and low noise: define the return path (counterpoise/radials) and choke the feedline.
Height and Band Dependence (EFRW)
- Very low feed end: strongest coupling to ground/objects → tuning and SWR drift easily with environment.
- Higher feed end: usually more repeatable, but impedance is still highly band dependent by nature of the design.
Summary — What Height Changes (and What It Doesn’t)
Instead of “fixed impedance at fixed height”, this table reflects reality: height effects depend on band (λ) and geometry.
| What you change by moving the feed end | EFHW (49/68/70:1) | EFOC (4:1) | EFRW (9:1) |
|---|---|---|---|
| Feedpoint impedance & SWR | Often very sensitive (high‑voltage end), especially on 160/80. | Moderately sensitive; usually more forgiving. | Very sensitive and band dependent; tuner is assumed. |
| Resonance shift | Common: end capacitance to ground/objects changes with height and routing. | Present, usually smaller. | Not a “resonant” design; tuning points move naturally. |
| Common‑mode on coax | High risk without strong choking + a defined counterpoise. | Lower risk, but still needs good choking practice. | High risk unless counterpoise + choke are defined. |
| Radiation efficiency | Mostly driven by losses + where current maxima are; not by feedpoint height alone. | Often excellent at modest heights on lower bands due to lower feed voltage. | Varies widely; depends heavily on counterpoise/ground and ATU losses. |
| Pattern / takeoff angle | Governed by band + geometry + average height of current maxima; higher bands can be very directional. | Similar physics; often fewer extreme surprises on intended bands. | Installation dependent; feedline often participates if not controlled. |
Conclusion
Feedpoint height is real — but it’s not a fixed requirement. It changes impedance and common‑mode behavior, and it changes differently per band because the same height becomes a different fraction of a wavelength from 160 to 10 m.
For maximum real‑world efficiency, EFHWs shine as monoband or octave dual‑band designs (160/80, 80/40, 40/20). That’s why RF.Guru focuses production on those combinations (and 40 m & 20 m monoband): they are the most repeatable, efficient EFHW solutions in practice.
Mini-FAQ
- Does an EFHW need to be high? — No fixed number. Feedpoint height mainly changes impedance and common‑mode behavior. Radiation depends on where the current maxima are and on losses.
- Why are octave dual‑band EFHWs typically the most efficient? — Because the higher band is the 2nd harmonic of the lower band (½λ → 1λ). The end remains a voltage maximum on both bands, so matching stays in a favorable region without forcing heavy reactive tuning.
- Why isn’t a “160–10 m EFHW” usually ideal on 17/15/12/10 m? — You can often tune it, but system efficiency is usually worse due to reactive stress, common‑mode risk, and multi‑lobed patterns with deep nulls.
- Do I always need a counterpoise and choke? — In most end‑fed systems, yes. A defined return path plus good feedline choking is the difference between repeatable behavior and “everything changes when it rains.”
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