EF-OCF: Why I Put the Coax Exterior to Work

I do not describe the coax as a counterpoise to excuse accidental RF on a feed line. I describe it that way because, in this architecture, a controlled section of shield exterior is intentionally recruited as the second antenna branch. If that conductor is missing from the drawing, the antenna description is incomplete.

End-Fed Does Not Mean One-Conductor

Current cannot leave a source terminal without a return path. At an end-fed antenna feedpoint, the return may be a dedicated wire, a radial or counterpoise system, the exterior of the feed line, a mast, station wiring, displacement current through the surroundings, or a mixture of these paths.

An EFHW and an EF-OCF can both look mechanically simple because only one long wire is obvious. Neither becomes electrically complete by hiding the other current path. The useful comparison therefore includes every conductor carrying current—not just the visible radiator length.

This is also how a shorter main span can be useful without pretending to abolish antenna size. I can divide the radiating structure between a main wire and a return branch that fits another part of the site. That is a layout advantage, not proof of equal electrical length, bandwidth or pattern for every EFHW and EF-OCF installation.

In the EF-OCF arrangement discussed here, the return path is made deliberate. The main wire is one asymmetric branch. The outside surface of a chosen coax section is the other branch. Its far end is defined by a common-mode choke. That is why I call the coax exterior the counterpoise or short OCF branch.

Three Currents Can Exist on One Coaxial Cable

The wanted transmission-line mode consists of equal and opposite currents on the centre conductor and the inside surface of the shield. Those fields are largely confined within the coax. A separate current can flow on the outside surface of the shield. Roy Lewallen's balun experiments describe that exterior current as imbalance or common-mode current and show why it can radiate independently of the internal pair.

In an ordinary feed line, we usually try to suppress that exterior current. In this EF-OCF section, we deliberately use it as an antenna current up to a chosen boundary. Calling it intentional does not change the physics: the route, height, surrounding conductors, soil, mast and choke impedance all determine how much current flows and where it goes.

The Choke Defines a Boundary, Not a Perfect Wall

Moving the choke changes the geometry and electrical loading of the intentional exterior branch. It can therefore change feed impedance, current distribution and pattern. This is not a coax-length trick applied after the antenna; the choke position belongs to the antenna design itself.

A real choke has finite, complex and frequency-dependent common-mode impedance. It does not make the downstream coax perfectly quiet on every band. Current can pass through the finite impedance or be re-excited farther along the line by coupling to the radiator, mast or station wiring.

That is why a physical distance alone cannot certify the boundary. Measure exterior current before and after the choke, farther down the cable and at the station. Repeat on every operating band and after changing the cable route or nearby conductors.

The 4:1 Ratio Follows the Installed Load

Moving the source away from the extreme end of a wire can place selected current modes in a more moderate impedance region. When measurements support it, a nominal 4:1 impedance transformation can be a practical starting point. It does not guarantee a 50 Ω input across multiple bands.

The real transformer sees a frequency-dependent complex load. Magnetising impedance, leakage inductance, winding capacitance, conductor and core loss, load phase, frequency, power, duty cycle and temperature all affect loss and stress. A lower numerical ratio is an engineering opportunity, not proof of a cooler or more efficient assembly.

Here is why I want that opportunity. For an ideal transformer, a 4:1 impedance ratio corresponds to a 2:1 turns and voltage ratio; 49:1 corresponds to 7:1. If an antenna geometry offers a suitable moderate load, I do not need the same voltage step-up as an extreme-end high-impedance feed. In a comparable conventional winding design, retaining adequate primary turns while reducing the required secondary ratio can reduce the amount of winding that has to behave properly over HF. Shorter winding paths can ease conductor-loss and parasitic-design burdens. That is a useful design direction, not a measured efficiency figure.

The distinction matters: fewer turns by themselves do not mean less core flux. At a given applied voltage, frequency and core area, reducing the relevant magnetising turns increases flux swing. Nor does the antenna-terminal voltage alone determine flux in every transformer topology. The low-frequency magnetising requirement still has to be met, and the high-frequency winding arrangement still matters. My preference is to reduce the required transformation by choosing the current system intelligently—not to remove turns indiscriminately.

RF.Guru keeps transformation and common-mode control separate because the two jobs require different evidence. That default suits the normally unbalanced field installation and can also serve a genuinely balanced case only when the combined UNUN-plus-choke network is deliberately arranged and verified as a hybrid interface.

Why This Architecture Can Be a Better Starting Point

The EF-OCF can be preferable when it turns an accidental return path into a declared antenna branch. You can route that branch, choose its boundary, measure its current and keep the station side out of the radiating system. An EFHW can also be built with a deliberate return structure, but too many installations leave that job to whatever coax, mast and wiring happen to be connected.

The second advantage is a less demanding matching target on the bands where that moderate load exists. It gives the designer room to pursue lower loss and useful bandwidth without the high step-up required at an extreme-end voltage maximum. A well-designed high-ratio transformer can still be efficient over its intended range; an unsuitable 4:1 can be poor. I choose the easier electrical job when the site and operating bands let me, then check the completed network rather than award it efficiency by name.

The final comparison is directional. The system that places more accepted power into the useful lobe can be better for a path even if its feedpoint SWR is less attractive. Conversely, a controlled return path can still produce the wrong elevation or azimuth pattern. Compare complete accepted-power patterns, not only transformer ratios or shack SWR.

The Coax Route Is Part of the Radiation Pattern

Once the coax exterior carries intentional current, it is a radiator. A vertical drop, a horizontal run, a sloping section and a cable laid near soil are different antenna geometries. They can show similar impedance while producing different current distributions and fields.

Internal coax velocity factor is not the cut-length rule for this exterior mode. The exterior current interacts with the jacket, soil, mast, supports and nearby conductors. Establish the useful branch length from the installed impedance and current distribution, then include the complete conductor geometry in the model.

Burial or enclosure is not automatically forbidden, but it changes dielectric loading, loss and coupling. If the intended branch is buried, run through conduit or placed against metal, that condition must be designed and measured rather than treated as invisible feedline routing.

A Separate Wire Is an Alternative Branch

A dedicated counterpoise wire can replace the intentional coax-exterior branch. In that arrangement the choke may belong near the feedpoint so the downstream coax is excluded as much as practical. The wire still needs a defined route, current map and pattern model.

The concrete benefit is freedom to route the antenna branch independently of the cable's journey to the station. If the coax must disappear into the ground, follow metalwork or enter the shack through an inconvenient route, I would rather place a dedicated return wire in a useful position and suppress exterior coax current near the feedpoint. That does not automatically increase efficiency or clean the pattern, but it separates two incompatible routing jobs instead of forcing one cable to satisfy both.

Compare EF-OCF and EFHW at the Same Boundary

Use the same source plane, accepted power, site and environmental state for both antennas. A tuner finding low SWR, a receiver sounding quieter or a transformer staying cool at one power level cannot rank complete systems by itself.

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