EFOC29 on 80 Metres: Geometry, Matching and Current Boundaries

The EFOC29 is a complete antenna system: approximately 29 m of radiator wire works with about 12 m of intentionally radiating coax exterior, terminated by a common-mode choke. Together they form the long and short current paths of the off-centre-fed architecture; installation height, routing and surroundings provide the final tuning variables.

The EFOC29 Architecture

The RF.Guru installation guide specifies roughly 28–30 m of radiator wire, a 4:1 unun and the first 11–12 m of coax braid as the counterpoise, followed by a choke. In that architecture the coax exterior is deliberately recruited as a radiating return conductor before the choke. Currents inside the coax remain the differential transmission-line mode; current on the outside of the shield is a separate common mode.

That distinction matters. A conventional OCFD has two unequal wire arms and normally uses a current balun to keep the feedline exterior out of the antenna. The EFOC29 system described here instead has one 29 m wire and an intended exterior-shield return section. Calling both arrangements “29/12 m off-centre-fed” hides materially different geometry, loss, pattern and safety boundaries.

How the 29 m + 12 m Geometry Works

Using the defined speed of light and ignoring end effects, 41 m is one free-space half wavelength at about 3.656 MHz. That is the useful first-order idea behind combining a 29 m wire with about 12 m of return conductor.

The table shows why the intentional return conductor is fundamental to the 80 m design. A 29 m conductor by itself has a free-space half-wave frequency near 5.17 MHz; adding the exterior-shield current path creates the composite length used around 3.65 MHz. The outside-shield wave is governed by its exterior fields, routing, height, ground and nearby materials rather than the coax manufacturer's internal dielectric velocity factor.

Real resonant length shifts with wire insulation and diameter, end capacitance, transformer and choke parasitics, bends, height, soil, the relative orientation of the wire and coax, and every parallel return path. The 12 m figure is a starting geometry, not an electrical constant.

The Choke Creates a Finite Boundary, Not a Perfect Wall

A common-mode choke adds series impedance to current on the cable exterior. It does not create an ideal open circuit. Its useful impedance is complex and frequency dependent, while the common-mode source and load also vary by band.

The specified choke at the end of the intentional return section establishes the designed common-mode boundary. Its completed construction—not turns count alone—sets complex impedance, resonance, bandwidth, voltage stress and thermal behaviour. Coax type, winding diameter, spacing, capacitance and ferrite material all matter, so the qualified choke and its published operating conditions should be used as a complete assembly.

Any bond, mast, radial, control cable or station path connected before or around that choke can bypass or alter the intended boundary. Even after a suitable choke is installed, measure current farther down the feedline. “Twelve metres of counterpoise” is established by a measured current transition, not by a tape measure alone.

How the 4:1 Feed Supports the Design

The 4:1 transformer is selected for the EFOC29’s off-centre feed region. An ideal 4:1 impedance transformation divides the antenna-side impedance by four and maps a purely resistive 200 Ω load to 50 Ω. Installed impedance remains complex and frequency-dependent, so the table is a useful matching reference while final trimming follows the measured system:

Those rows deliberately omit reactance and transformer imperfections. Real results also depend on magnetising impedance, leakage inductance, winding capacitance, conductor and core loss, and the common-mode path. Fair-Rite's broadband-transformer guidance treats these as frequency- and load-dependent quantities; ratio alone supplies neither insertion loss nor power rating.

A tuner can present a suitable impedance to the transmitter, but a shack-end tuner does not remove standing waves or loss on the cable and transformer ahead of it. An internal tuner may or may not cover the measured impedance. Publish the tuner model, its range, the calibration plane and the complex load instead of saying that any small tuner will work.

Europe Changes the Operating Window, Not the Electromagnetics

ITU Region 1 allocates 3.5–3.8 MHz to the amateur service, and the IARU Region 1 HF band plan coordinates preferred use within that range. The band plan itself is operating guidance; national authorisations and licence conditions govern. The current Belgian BIPT table, for example, authorises 3.5–3.8 MHz but assigns different power conditions by certificate class.

United States terminology does not make 3.8–4.0 MHz a separate non-harmonic physical object. Current FCC Part 97 labels 3.500–3.600 MHz “80 m” and 3.600–4.000 MHz “75 m” for an Amateur Extra control operator in ITU Region 2, with narrower privileges for some licence classes. The labels describe regulatory segments. Antenna current modes depend continuously on frequency and geometry.

Retuning the lowest mode can shift higher-mode impedance curves, but the shift is not a promise that 40, 20 and 10 m will all become unusable. The exterior coax follows a different route from the main wire, transformer and choke parasitics are frequency dependent, and the structure spans multiple wavelengths on higher bands. Model and sweep every target band after each geometry change.

Propagation and Pattern Are Installation Questions

There is no “European 80 m propagation” that makes one antenna perfect. ITU-R P.533 predicts HF circuit performance from the two endpoints, frequency, month, time, solar activity, transmit power and both antennas. ITU-R P.372 also treats atmospheric, galactic and man-made noise as frequency-, location- and time-dependent inputs.

At 3.5–3.8 MHz, ordinary residential support heights are usually a small fraction of a wavelength. A mostly horizontal wire may then favour high elevation angles useful for regional skywave, while a vertical or sloping exterior-coax section changes polarization and the lower-angle field. Ground conductivity, height, slope, coax route and nearby conductors can move maxima and nulls.

On higher bands the composite conductors span multiple half wavelengths and develop additional lobes. “Harmonic alignment” can describe an impedance opportunity, but it does not guarantee gain toward a particular station. Use NEC or another validated full-wave model including the exterior shield and real ground, then verify with controlled field measurements.

Loss Budget for a Trap-Free System

A trap-free wire avoids trap loss, but the complete system still has:

• wire and connection resistance;
• ground and nearby-object loss from the installed fields;
• transformer core, conductor and dielectric loss;
• common-mode choke dissipation;
• matched and mismatch-related coax loss; and
• possible current in lossy unintended return paths.

Good SWR does not separate those mechanisms. Loss can make an input trace look broader and flatter. Measure transformer and choke loss or temperature under a representative complex load, calculate the actual cable loss, map exterior current and compare calibrated field strength at equal accepted power.

Power handling likewise cannot be inferred from “4:1.” Voltage is greatest near current minima; current and heating concentrate elsewhere. Verify the exact transformer and choke with the intended frequency, complex load, PEP, average power, waveform, duty cycle, ambient temperature, enclosure and mismatch. Stop a power ramp on abnormal temperature, impedance drift, arcing or insulation stress.

Legal Power and RF Exposure

Do not convert “Europe” into one band, power or exposure rule. Check the current national authorisation, certificate class, emission and any ERP/EIRP or transmitter-power convention. The IARU band plan does not replace those conditions, and this article is not individual legal advice.

The intentional exterior-coax current also matters to exposure. ICNIRP's 2020 guidelines cover 100 kHz–300 GHz, but enforceable procedures come from the applicable jurisdiction. At HF, near-field electric and magnetic fields, contact current, distance, geometry, duty cycle and averaging can matter. A radiating coax section routed near people, equipment or accessible metal must be included in the assessment; a far-field EIRP shortcut may not describe that region.

A Reproducible Installation Test

1. Record the complete geometry. Include the 29 m wire, insulation, height, slope, transformer, coax make and route, choke location, mast, bonds and station wiring.
2. Measure complex impedance at the feed assembly. Calibrate or de-embed to that plane and save resistance, reactance and uncertainty, not only SWR screenshots.
3. Characterise the 4:1 network. Measure insertion loss and impedance with realistic complex loads across all target bands; repeat after thermal stabilisation at representative power.
4. Measure the choke. Record complex common-mode impedance versus frequency in the final winding and check voltage, current and temperature.
5. Map exterior current. Measure along the first 12 m, immediately beyond the choke and at the shack. Repeat after moving bonds or changing coax route.
6. Separate line and tuner effects. Use the actual cable attenuation and measured load; record whether a tuner is internal, shack-end or remote.
7. Model the complete radiator. Include the exterior shield, ground and surrounding conductors; compare impedance and patterns after geometry changes.
8. Verify radiation. Use rapid calibrated A/B field substitution at equal power from a stated reference plane. Receiver reports and QSOs alone cannot isolate antenna gain from propagation.
9. Check safety and legality. Use the actual authorised power, mode duty cycle, accessible geometry and current national assessment method.

Bottom Line

The EFOC29 is a deliberate 80 m antenna architecture: roughly 29 m of wire plus a bounded exterior-coax return path creates a nominal composite length near half a free-space wavelength around 3.65 MHz. The 4:1 transformer and choke complete that controlled current system.

Its European emphasis comes from centring the design near the Region 1 operating window. Useful bandwidth, tuner requirement and pattern are then commissioned for the actual cable route, height, ground and surroundings. Treat the wire, coax exterior, transformer and choke as one antenna system, preserve the intended current boundary, and use measured impedance and exterior current for the final installation adjustments.

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