Designing the EFOC56 — A 160/80 m Flattop or Inverted-L
Updated for the RF.Guru 4:1 high-power voltage transformer platform (8 kW CCS).
There is no doubt about it: our EFHW 160/80 Inverted-L remains the most efficient antenna solution for serious top-band work. It places more current higher in the air and behaves much closer to a classical vertical with a well-defined return system. The downside is obvious — it needs space.
The EFOC56 exists for operators who want reliable and predictable 160 m and 80 m performance, but who are constrained in height, span, or installation freedom. It is an OCF-style, voltage-fed flattop or inverted-L, built around our new 4:1 voltage transformer rated for 8 kW CCS, and intentionally optimized for 160 m / 80 m with usable behavior on 40 m.
What the EFOC56 Is (and Is Not)
The EFOC56 is not a balanced antenna, and it does not attempt to be one. Like every off-center-fed design, current maxima and minima are intentionally displaced. That is precisely why a voltage transformer is used instead of a current balun.
- Voltage-fed OCF topology
- Single long radiator plus one defined return path
- 4:1 voltage transformer at the feedpoint
- Mandatory 1:1 current choke to control feedline current
This is the same controlled-imbalance design philosophy used in the EFOC29.
Electrical Length Strategy
The antenna is designed around a half-wave on 160 m:
L (m) ≈ 143 / f (MHz)
That electrical length is intentionally split approximately 70 % / 30 %:
- ~70 % long radiator (vertical + horizontal)
- ~30 % counterpoise / return section
This mirrors the classical “one-third from the end” OCF concept, but applied to a low-band inverted-L where height, capacitance to ground, and current distribution dominate behavior.
Mechanical Geometry
- Vertical section: 12–16 m
- Horizontal section: ≤ 53 m
- Return path: counterpoise wire or controlled coax shield section
With this geometry, the EFOC56 fits locations where a full-size EFHW 160/80 is simply not possible.
Using the Coax Shield as Counterpoise
Yes — the counterpoise can be intentionally integrated into the coaxial feedline. In this configuration, the outer braid of the coax becomes the electrical return path for the short (~30 %) section of the antenna.
Electrically, this is equivalent to using a separate counterpoise wire — provided the choke is placed at the correct electrical position.
- The physical counterpoise wire is omitted
- The coax shield up to the choke forms the return path
- The choke position defines where the antenna ends and the feedline begins
When using the coax as counterpoise, electrical length matters. Account for velocity factor — not connector-to-connector distance.
160 m Cut-List (with 80 m Harmonic Reference)
All lengths include +2 % for trimming. Always cut long.
| 160 m f (MHz) | 80 m equiv (MHz) | Long radiator CUT (m) | Counterpoise CUT (m) | Total CUT (m) | Horiz if V = 12 m (m) | Horiz if V = 16 m (m) |
|---|---|---|---|---|---|---|
| 1.800 | 3.600 | 56.72 | 24.31 | 81.03 | 44.72 | 40.72 |
| 1.825 | 3.650 | 55.95 | 23.98 | 79.92 | 43.95 | 39.95 |
| 1.830 | 3.660 | 55.79 | 23.91 | 79.70 | 43.79 | 39.79 |
| 1.850 | 3.700 | 55.19 | 23.65 | 78.84 | 43.19 | 39.19 |
| 1.875 | 3.750 | 54.45 | 23.34 | 77.79 | 42.45 | 38.45 |
| 1.900 | 3.800 | 53.74 | 23.03 | 76.77 | 41.74 | 37.74 |
| 1.950 | 3.900 | 52.36 | 22.44 | 74.80 | 40.36 | 36.36 |
| 2.000 | 4.000 | 51.05 | 21.88 | 72.93 | 39.05 | 35.05 |
Efficiency vs Footprint — an Honest Trade-Off
- EFHW 160/80 Inverted-L: higher efficiency, larger footprint
- EFOC56: slightly lower efficiency, far more installation flexibility
If you have the room, choose the EFHW. If you do not, the EFOC56 delivers controlled, repeatable, and predictable low-band performance without pretending physics can be cheated.
Mini-FAQ
- Is the EFOC56 balanced? — No. It is intentionally voltage-fed and off-center.
- Do I need a choke? — Yes. A 1:1 current choke is mandatory.
- Can the counterpoise be integrated into the coax? — Yes. The coax shield can act as the counterpoise.
- Where does the choke go in that case? — At the electrical distance defined by the counterpoise length in the cut-list table.
- Does this reduce efficiency? — No, provided the choke is correctly positioned.
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