EFOC29 Get 80m and 30m Under Control: Practical Tips for Better SWR

The EFOC29 is a compact off-center-fed antenna with a 29-meter wire, a 4:1 UNUN, and a 1.2-meter counterpoise implemented via the coax braid, paired with a choke placed at 12.2 meters. It performs well across many HF bands, but users sometimes struggle with high SWR on 80 meters and 30 meters. Here's why that happens and what you can do to fix it.

Why 80m and 30m Are Problematic

  • Harmonic mismatch: The 29-meter wire length doesn’t align neatly with 1/2 or 1/4 wavelengths on 80m or 30m.
  • Impedance excursions: Feedpoint impedance can drift far outside the transformation range of the 4:1 UNUN.
  • Common-mode coupling: Especially on lower bands, the coax may radiate or pick up noise, distorting the measured impedance.

Reposition the Choke

The factory-recommended 12.2-meter location is a good general compromise, but:

  • Try placing the choke closer (e.g. 8 m) or farther (e.g. 14 m) from the feedpoint.

  • Optimal choke position varies based on your coax length, height, and layout.

Reference the Manual for Wire Tuning

The installation manual specifies that the 80m dip is tuned just below 3.5 MHz. Any significant deviation from the intended 29-meter length can degrade the tuning. Trimming or extending the radiator is not advised unless you are intentionally retuning the antenna for a fixed-frequency application.

Height Matters

Feedpoint height and wire geometry can have a significant effect on impedance:

  • Raising or lowering the feedpoint by even 1–2 meters can shift the impedance curve.
  • Keep the wire as horizontal as possible. Avoid steep slopes or tight bends.
  • When space is limited, aim for a gradual slope and at least 6–7 meters average wire height.

Add a Resonance-Stabilizing Stub (Advanced)

A parallel stub tuned to 80m can help flatten SWR excursions without affecting other bands:

  • Use a 2–3 meter length of RG58, open at one end, connected near the feedpoint.
  • Properly tuned, it can target problem impedance zones without disturbing other bands significantly.

Final Advice: Don’t Fear 3:1

  • Most tuners easily handle 3:1 SWR.
  • Focus on ensuring the antenna radiates well and keeps noise low.
  • Always inspect R and X separately—a high SWR isn't always a bad match.

Summary

If you're struggling on 80m or 30m with an EFOC29, start by moving the choke, double-checking your radiator length, and optimizing the feedpoint height. Consider a stub if you're chasing a flatter 80m curve. The system is well-designed and usually only needs minor tweaks.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.