Inverted-L EFOC29: Why 10 m and 12 m Don’t Line Up Like in Sloper or Flat-Top Versions
The EFOC29 is a versatile off-center fed antenna with a 29 m radiator and a 4:1 unun. In its classic sloper or flat-top configurations, the harmonic structure aligns well with most HF bands from 80 m through 10 m. But when the same design is installed as an Inverted-L, operators often notice that 12 m and 10 m “fall out of place.” Here’s why.
How the Inverted-L Changes Current Distribution
Unlike a flat-top or sloper, the Inverted-L has a vertical section and a horizontal section. The vertical portion favors low-angle radiation and shifts the current distribution. This means the harmonic resonance points no longer line up neatly with the expected multiples of the 29 m wire length.
- Sloper/Flat-top: current distribution is predictable, close to a half-wave and its harmonics.
- Inverted-L: mixed vertical/horizontal current paths “bend” the harmonic series, shifting effective electrical length on higher bands.
Why 10 m and 12 m Are Affected
On mid-HF bands like 40 m, 20 m, and 15 m, the geometry shift is still forgiving — the antenna resonates within tuner range. But on 12 m and 10 m:
- The 29 m wire length corresponds to a non-integer multiple of half-waves at these frequencies.
- The vertical rise increases effective capacitance, pulling resonance away from the neat harmonic slots that exist in flat-top or sloper geometries.
- Instead of a usable 2–3:1 SWR, operators see 6–9:1 on 10 m/12 m, even with a good choke installed.
This is not a “fault” in the unun or counterpoise — it’s geometry. The Inverted-L inherently bends the current envelope, and higher bands are the first to reveal the mismatch.
Can It Be Fixed?
Yes, but not without trade-offs:
- Add a parallel wire element cut for 10 m or 12 m (a simple “fan” addition).
- Use a separate vertical for 10 m/12 m if those bands are critical for you.
- Accept tuner use — many modern radios/tuners can match 6:1 SWR at QRP or moderate power, but efficiency suffers.
Mini-FAQ
- Does the Inverted-L always miss 10 m/12 m? — Almost always. The vertical/horizontal mix distorts harmonic alignment on these upper bands.
- Is efficiency lower on 10 m/12 m? — Yes, especially with tuner-only operation. Efficiency is much better on 40 m, 20 m, 15 m.
- Will a choke help? — A choke is essential to suppress common-mode, but it won’t fix band misalignment caused by geometry.
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