160/80 m EF-OCF or EFHW: Which Fits the Site?

My starting point is not “which bend looks nicer?” It is the operator who would like a full-size 160/80 m EFHW inverted-L but cannot give its long radiator the height, clear span or support arrangement it needs. That is the reason to consider an end-fed off-centre arrangement: keep convenient feed access while dividing the useful current path differently.

If I can install the long EFHW wire high and clear, with the right matching and return arrangement, I have no reason to abandon that good installation merely because another architecture exists. If fitting it means dragging a large section down beside the house or folding it tightly around obstacles, an EF-OCF with a shorter main branch and a deliberately routed return branch becomes a serious alternative. The advantage is usable installation geometry, not an exemption from the electrical size of a low-band antenna.

The Real Choice: One Long Branch or a Deliberate Split

In the EFHW case considered here, a main wire near a half-wave on 160 m is fed close to its end and bent into an L. It still needs another side to the feed circuit: a deliberate return arrangement and common-mode control. “End-fed” does not mean current disappears at one terminal.

The EF-OCF alternative moves the feed to an off-centre point in the complete radiating path. A substantial portion belongs to the shorter return branch rather than to the long elevated wire. That branch may be a separate conductor or an intentionally used section of coax exterior. The feed connection can remain near a convenient support even though it is not at the electrical end of the complete antenna.

This is redistribution, not miniaturisation. Moving part of the conductor into the return route can save elevated main span; it does not remove that conductor from the RF problem. A route down a clear support and away from the station may be useful. A route pressed against lossy material or tangled with house wiring may defeat the benefit. The return route must earn its place in the design just as the main wire does.

Tom Rauch, W8JI's end-fed analysis is useful here because it explicitly includes the counterpoise, matching network and feedline current. His off-centre-fed discussion also distinguishes the feed tap from the wire's natural current distribution. The installation comparison must include both branches; changing the feedpoint is not by itself a guarantee that current moves to a more useful place.

Design the EF-OCF for Both Low-Band Modes

A useful first model begins with a total conductor path near a half-wave at the chosen 160 m frequency. In free space, the half-wave reference is c/(2f); the installed resonant length is changed by conductor diameter and insulation, height, bends, end capacitance, ground, nearby structures and the return path. A handbook shortening factor is a starting cut, not a finished dimension.

At roughly twice the frequency, the same path can support a full-wave mode near 80 m. That does not guarantee that the two resonances land in the desired parts of both amateur bands. End effects are frequency dependent, the bend couples differently to ground on each band, and the matching and return structures add their own reactance. Treat 160 m and 80 m as two installed modes that must both be designed.

The higher mode also contains more current maxima and a phase reversal along the wire. Its feed impedance and radiation pattern can therefore differ sharply from the 160 m fundamental even when both bands produce a usable SWR.

The Off-Centre Split Is a Design Variable

There is no universal 70/30 split for this antenna. Moving the feedpoint changes the current and voltage presented to the matching network on both bands. It also changes how much conductor belongs to the long branch and how much belongs to the explicit return branch.

A candidate split should be evaluated against the complex impedance on 160 m and 80 m, the current and voltage at the feedpoint, the intended return geometry and the pattern created by both branches. A point that gives a convenient resistance on one band may present large reactance, voltage or common-mode current on the other.

Off-centre geometry by itself does not make every OCF antenna common-mode. A conventional two-wire OCF dipole can still carry equal and opposite terminal currents in its intended differential mode. The one-end EF-OCF architecture discussed here is deliberately asymmetric because the two branches are implemented differently and may use the coax exterior as one radiating branch. Differential transformation and the downstream common-mode boundary must therefore remain separate design functions.

Then Choose the Shape: Flattop

A flattop keeps most of the current path horizontal. It is attractive when two suitably high supports and a clear span are available. On the 160 m fundamental, a simple horizontal half-wave begins with a broadside pattern, but its elevation response depends strongly on height in wavelengths and ground. At the modest heights available on many properties, a large high-angle component is common.

On 80 m, the full-wave mode adds lobes and nulls. Feed asymmetry, the return branch, unequal support heights, bends and nearby conductors can rotate or fill those lobes. The word “flattop” describes the drawing; it does not certify a pattern.

A mostly straight, level flattop gives a simpler geometry to describe than a branch with several bends and height changes. It still needs the clear span and support strength for its main branch. The EF-OCF's split does not make those mechanical needs disappear; it changes how much of the current path must occupy that span.

Or Bend the Main Branch Into an Inverted-L

An inverted-L uses a vertical section and turns the remaining conductor horizontally. That can fit a long low-band wire on a site with one strong support and less horizontal room. The vertical and horizontal sections are not two separate antennas: their fields combine according to current magnitude, phase, orientation, height and ground reflection.

The bend does not guarantee a low take-off angle. If a current-rich region falls in the vertical section, that section can contribute useful vertically polarised radiation. If the current maximum falls well into a low horizontal section, high-angle radiation and environmental coupling may dominate. On 80 m, additional current maxima and reversals make the answer even more installation specific.

For the space-constrained installation, the useful opportunity is to put more of the available main branch in clear air rather than forcing a longer branch through poor surroundings. Choose the vertical height and bend position by plotting current magnitude and phase on both bands, then inspect the complete azimuth and elevation patterns. A feedpoint match cannot reveal whether that opportunity produced useful radiation in the desired direction.

The Return Branch Is Part of the Radiator

An end-fed off-centre wire is a two-terminal RF system. The shorter branch may be a separate conductor, or the exterior of a declared coax section may be used intentionally between the transformer and a choke. In the latter arrangement, differential current continues inside the coax while the outside of the shield carries the intended antenna current.

This is the practical price of the shorter main span: the declared return branch now carries intentional antenna current and needs RF clearance. Using the coax exterior can avoid hanging a separate return wire, but it does not create a second, independent space-free antenna. The choke is the intended end of that exterior-current branch. It creates an impedance boundary; it is not a perfect wall. Its position and common-mode impedance affect resonance, feed impedance, current distribution and pattern. Map exterior current on both bands and confirm that the station-side coax, mast and wiring no longer become uncontrolled extensions of the antenna.

The Transformer and Choke Have Different Jobs

The transformer handles the differential impedance presented by the two antenna branches. When the measured complex load supports it, a nominal 4:1 UNUN can be a sensible starting network. A 4:1 label does not promise 50 Ω on either band, cancel reactance or establish balance.

The separate 1:1 choke addresses current on the outside of the coax and other connected conductors. Specify it by complex common-mode impedance, voltage, current, temperature and bandwidth at the chosen position. A low differential SWR is not evidence that the choke has created the intended boundary.

Design both components for the actual complex loads. Transformer magnetising current, leakage, winding capacitance, conductor and core loss, and the choke's own resonance all change with frequency. No power rating can be transferred from a core name or turns count to the completed, enclosed assembly without electrical and thermal qualification.

Commission the Geometry Before Trimming the Final Length

• Fix the site model: record support heights, bend position, horizontal direction, conductor, insulation, nearby metal, buildings, trees, ground and every intentional return conductor.
• Choose candidate branch splits: calculate or model complex feed impedance, current and voltage on both bands rather than copying one percentage.
• Include the real feed structure: model the transformer terminals, enclosure, intentional coax exterior, choke position, downstream coax route and station bonds.
• Install long: leave practical trimming margin, but trim against both target modes and the final geometry rather than a universal cut table.
• Measure at declared reference planes: record complex impedance at the antenna terminals and after the matching network, then separate mismatch from network and feedline loss.
• Map exterior current: check the intended branch and the station-side feedline at repeatable positions on 160 m and 80 m.
• Check the operating result: compare accepted power, thermal behaviour, current distribution and the pattern or field paths that matter for the station.

Choose the Shape From the Site and the Required Pattern

Neither shape is inherently more efficient. Conductor and ground loss, matching-network loss, return-current control, feedline loss and current distribution decide the accepted-power budget. Geometry then decides where the radiated power goes. Compare the complete installed systems rather than ranking the names.

Keep the Full EFHW Where It Fits; Use the Split Where It Helps

My preference is straightforward. If the site accommodates the full-size EFHW inverted-L with its long radiator high and clear, build that arrangement properly and keep its matching and return path under control. There is no need to introduce a more substantial radiating return branch merely to follow a different label.

If the main span is what prevents a sensible installation, the EF-OCF approach gives another degree of freedom: move part of the intended current path into a return route that the site can accommodate. When that lets the main branch stay higher, clearer or less tightly folded, it solves a genuine installation problem. This is why I consider it—not because a transformer can make a short piece of wire behave as though the missing antenna were still there.

The trade is not a fixed efficiency penalty or a fixed gain bonus. A clear return branch can be a useful part of the radiator; a poorly placed one can add loss, coupling and unwanted pattern changes. Both architectures must carry the current somewhere. I would rather make that route deliberate than force a nominally larger antenna into unsuitable surroundings and call the job finished.

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