EFOC: Why “the coax is the counterpoise” — and why it can beat an EFHW

Most hams equate “end-fed” with EFHWs and 49:1 transformers, but an EFOC behaves very differently. It is essentially an off-center-fed dipole where the short leg is the intentional coax-shield radiator between transformer and choke, and the long leg is the wire. By controlling exactly how much coax is allowed to radiate — and where it stops — you get a predictable, stable, low-voltage, multiband antenna that often outperforms a classic EFHW in real installations.

EFHW vs OCF: apples-to-apples on actual size

End-fed does not mean “short.” Electrically, EFHW and OCF designs use almost the same total conductor length:

• EFHW (80 m): ~40 m wire + 1–3 m return path (counterpoise or coax braid) → ≈ 41–43 m total.
• OCF dipole (80 m): typical split 29 m + 12 m = 41 m total.

The span looks different, but the electrical length is essentially the same. The EFHW and the EFOC simply hides part of its radiator in the return path.

Why the OCF can be more forgiving

The difference is where the antenna places its voltage and current peaks.

• EFHW — end-fed at a voltage maximum, very high feedpoint impedance (2–4 kΩ), high E-field, high ferrite stress, and highly sensitive to gutters, walls, and wet branches.
• OCF — fed near a current maximum, lower voltage, a few hundred ohms impedance, far less reactive behavior, and more tolerant of real-world surroundings.

The EFOC applies these OCF advantages to an end-fed layout.

EFHW vs EFOC: resonant vs near-resonant (and why it matters)

An EFHW wire is cut to a true half-wave on its lowest band, making it fully resonant on that band and its harmonics. The end-feedpoint sits at a voltage maximum with very high impedance — typically 2–4 kΩ. That’s why EFHWs require 49:1 or 64:1 transformers, run high RF voltages (hundreds of volts RMS at 100 W), and can be easily detuned or lossy if ferrites and insulation are not ideal.

An EFOC, by contrast, shifts the feedpoint to roughly 20–25% from one end, creating a near-resonant feedpoint around 150–300 Ω. A simple 4:1 unun usually brings this close to 50–75 Ω. Voltage stays modest, ferrite stress is lower, and the antenna tends to be broad, stable, and forgiving across multiple bands.

Bandwidth tendency: An EFOC’s lower transformation ratio and lower feedpoint impedance mean a lower system Q, which often produces a wider usable SWR bandwidth on each band than a classic EFHW. But this is a tendency, not a rule — wire thickness, ground, height, loading, ferrite type, turns, and common-mode control all shape the real bandwidth.

Bottom line: EFHW = resonant, high-impedance, voltage-fed. EFOC = near-resonant, lower-impedance end-feed with easier matching, lower stress, and often wider multiband usability. And remember: resonance does not equal efficiency. What determines your EIRP is pattern + loss, not whether the feedpoint sits exactly on resonance.

What an EFOC (EC-OCFD) actually is

An EFOC is an off-center-fed dipole consisting of:

• A 4:1 transformer at the feedpoint
• A long wire (main radiator)
• A short coax-shield leg (transformer → choke)
• A 1:1 choke placed precisely to stop radiation

Typical short-leg distances:

• 3.7 m (40–6 m EFOC)
• 12.2 m (80–10 m EFOC)

Before the choke, the coax is a radiator. After the choke, the coax becomes “quiet.” That is the defining principle.

The short piece: why the coax section is the EFOC’s short OCF leg

In a traditional OCFD, the short leg is just wire. In an EFOC, the short leg is the coax shield between transformer and choke. This is intentional.

• The transformer sits at an OCF feedpoint (~20–25% position).
• The coax shield becomes the electrically short leg.
• The choke defines where that leg ends.

Move the choke → you change the short-leg length → you change the feedpoint impedance. This is not a flaw — it is the tuning mechanism.

Lower feed voltage than an EFHW

An EFOC’s ~200 Ω feedpoint at 100 W:

• 0.7 A RF current
• ~140 V RMS

An EFHW’s ~3 kΩ feedpoint at 100 W:

• ~0.2 A
• ~500–550 V RMS

Much lower voltage means lower losses, less heating, less detuning, and no need for heroic ferrite stacks.

Why an EFOC can beat an EFHW in real installations

Lower transformer loss

A 4:1 transformer is dramatically less stressed than a 49:1 — fewer turns, less leakage inductance, and lower voltage.

Defined return path = quieter receive

An EFHW often radiates along its entire feedline unless perfectly choked. An EFOC forces radiation to stop: predictable pattern, quieter SNR.

Less high-voltage sensitivity

EFOCs couple less into gutters, siding, walls, and wet foliage. They behave better in small gardens.

Shorter wire, still multiband

• 40–6 m: ~17 m wire
• 80–10 m: ~29 m wire

Vertical or horizontal without harming performance?

The long wire does almost all the radiating. The short coax leg simply sets the feedpoint. The long wire may be deployed as:

• Flat-top
• Sloper
• Inverted-L
• Inverted-V

As long as the choke sits at the correct position, the antenna behaves predictably.

Burying feedline?

• Transformer → choke: NO (this portion radiates)
• After the choke: YES (coax is “quiet”)

A separate counterpoise wire: even more efficiency

If you do not want coax to be the short leg, you can replace it with a short insulated counterpoise. Benefits:

• Minimal common-mode current on coax
• Cleaner azimuth pattern
• Higher efficiency on 80/40 m
• Less interaction with nearby objects

When using a separate counterpoise, the choke moves directly to the feedpoint box.

Quick recipes

40–6 m EFOC

• Long leg: 16.8 m (55 ft)
• Short leg: 3.7 m (12 ft)
• Choke at 3.7 m
• Any coax length after the choke

80–10 m EFOC

• Long leg: 29 m (95 ft)
• Short leg: 12.2 m (40 ft)
• Choke at 12.2 m
• WARC bands require a tuner

TL;DR

• An EFOC uses the coax (before the choke) as the short OCF leg — by design.
• A choke defines where the leg ends and keeps the rest of the coax quiet.
• EFOC typically outperforms EFHW 80–10 m due to lower loss and lower feed voltage.
• Using a dedicated counterpoise further increases efficiency and stability.

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