6:1 Balun
A 6:1 current balun (more precisely 6.25:1) steps a roughly 300Ω feedpoint down to 50Ω coax impedance — useful on off-center-fed and Windom-style antennas where the feedpoint lands higher than the 200Ω a standard 4:1 balun assumes, which happens on some bands depending on the exact feed point and wire length. This guide covers the trifilar winding method that produces the 6.25:1 ratio, a turns calculator, and when to reach for 6:1 instead of 4:1.
Why 6:1 exists alongside 4:1
An off-center-fed or Windom-style antenna's feedpoint impedance isn't a fixed number — it depends on band, the exact feed point ratio, and wire length, and commonly ranges anywhere from about 150Ω to 300Ω+ across the bands a single antenna covers. A 4:1 balun assumes something close to 200Ω; on bands where the real feedpoint impedance runs closer to 300Ω, a 6:1 balun lands much closer to 50Ω and gives a cleaner match without relying entirely on an antenna tuner to absorb the difference.
Why it's really 6.25:1, not an exact 6:1
Practical current-balun designs get their ratio from a turns ratio, and impedance scales with turns ratio squared. A clean integer turns ratio close to 6:1 in impedance is 5:2 (2.5:1 turns ratio), since 2.5² = 6.25. Ham radio literature and manufacturers round this to "6:1" for simplicity, but the actual transformation is 6.25:1 — worth knowing so a 300Ω feedpoint maps to about 48Ω, not exactly 50Ω, which is well within normal SWR tolerance.
The trifilar winding method
Where a 4:1 balun uses two identical (bifilar) windings, a 6:1 balun uses three identical (trifilar) windings of equal turns, wound together through the core for tight coupling, then connected in series-aiding sections so the antenna side effectively sees 5 winding-turn units in series while the coax side sees 2 — producing the 5:2 turns ratio above. As with the 4:1 design, the same wound core that provides the impedance transformation also acts as a common-mode choke, since it's built the same way as any ferrite-core choke winding.
- vs. 4:1 bifilar: more complex tap arrangement, but the same underlying principle — turns ratio squared sets impedance ratio.
- Coupling matters more with three wires: keep all three conductors the same length and wind them together in one pass, not separately, to preserve tight coupling across the full HF range.
Sizing for power
Power handling follows the same core-selection logic as any other ferrite-wound current balun — see the Core Selection Guide for the flux density math. Because trifilar windings use more wire in the same window area, a slightly larger core (FT-240 size) is the practical default even at moderate power, simply to leave room for three conductors without overly cramming the winding.
| Core Configuration | Trifilar Turns | Approx. Winding Inductance | Recommended Power |
|---|---|---|---|
| FT-140-43 (single) | 4 | ~14 µH | Up to 100W |
| FT-240-43 (single) | 3 | ~10 µH | 100–400W |
| FT-240-43 (stacked x2) | 6 | High margin | Full legal limit (1.5kW) |
6:1 Balun Turns Calculator
Materials for a trifilar-wound 6:1 current balun
A 6:1 (6.25:1) trifilar current balun wound with 3 sets of turns on a single FT-240-43 core.
Building a 6:1 Trifilar Current Balun
The trifilar winding takes more care than a bifilar 4:1 — label wires before you start and double-check the tap connections before closing the enclosure.
Choose core configuration and turns
Use the reference table or calculator above to match core and turns to your power level — a single FT-240-43 at 3 trifilar turns covers most 100-400W installations.
Prepare and wind the trifilar set
Cut three equal lengths of wire and mark each with a different tape color at both ends (for example red, yellow, blue) before winding — with three conductors, mislabeling becomes far more likely than with a bifilar pair. Wind all three together through the core in a single pass for the target turn count, keeping the three wires in the same relative order at every turn.
Connect the 5:2 tap arrangement
Connect the three windings in series-aiding order (end of one wire to start of the next, preserving winding sense) so that the antenna-side terminals span all three windings in series (the "5" side of the ratio) while the coax-side connection taps in after the first two windings' worth of series turns (the "2" side). Connect the coax center conductor and shield across that 2-turn-equivalent tap point, and the two antenna terminals across the far ends of the full 5-turn-equivalent series chain.
Test with a dummy load before installing
Connect a non-inductive resistor close to 300Ω across the antenna terminals and sweep SWR at the coax input with a NanoVNA. Expect SWR in the same 1.5:1–1.8:1 range as a correctly built 4:1 balun would show into its own matched dummy load; if it's far outside that, recheck the tap connections before assuming an antenna problem.
Weatherproof and install at the feedpoint
Mount the tested balun in a sealed enclosure with a drip loop below the coax entry, and support the enclosure's weight independently from the antenna-wire terminal connections.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR high on all bands despite correct antenna dimensions | Trifilar tap arrangement wired incorrectly — most common error on this design | Test the balun alone with a ~300Ω dummy resistor across the antenna terminals; a correctly wound unit shows SWR under about 1.8:1 | Re-verify continuity from each terminal to its intended tap point against the 5:2 series arrangement described above; re-wind if any winding sense is reversed |
| Balun seems to work but the match isn't noticeably better than a 4:1 was | Actual feedpoint impedance on this antenna/band is closer to 200Ω than 300Ω, so 4:1 was already the better fit | Compare SWR curves with both a 4:1 and 6:1 balun on the same antenna if possible, or model/measure actual feedpoint impedance | Use whichever ratio measures the lower SWR for your specific antenna and feed point — there's no universal right answer between 4:1 and 6:1 for every off-center feed design |
| One or two bands show much worse SWR than the rest | A wire from the trifilar set is mislabeled or connected to the wrong tap | Recheck tape labeling against the actual winding order — three same-colored wires without distinct marking are easy to mix up mid-build | Re-trace each wire's winding path with an ohmmeter from a known terminal before re-soldering any suspect connection |
| Winding feels loose or wires shift inside the enclosure | Three-wire winding wasn't secured as tightly as a simpler bifilar winding would be | Inspect the winding for movement after normal handling/shipping vibration | Anchor the winding to the enclosure floor with a tie-wrap or hot glue at the core, away from any solder joints |
How do I know whether my antenna needs 4:1 or 6:1?
There's no way to know for certain without measuring or modeling — feedpoint impedance depends on your specific wire length, feed point ratio, and height above ground. If you already have a 4:1 balun and one or two bands show stubbornly high SWR that a tuner struggles with, trying a 6:1 balun on that same antenna is a reasonable experiment; some published multiband OCFD designs note better 40m results with 6:1 specifically.
Why is it called "6:1" if the real ratio is 6.25:1?
Ham radio convention rounds to the nearest whole-number label for simplicity, the same way "4:1" balun designs are sometimes built with turns ratios that give slightly-off-4 ratios in practice. The 0.25 difference (48Ω vs. 50Ω from a 300Ω feedpoint) is well within normal SWR tolerance and not something you need to correct for.
Is the trifilar winding significantly harder to build than a bifilar 4:1?
It takes more care, mainly in labeling and verifying the three-wire tap connections, but the physical winding process (wind all three wires together through the core) is not fundamentally more difficult. Budget extra time for continuity-checking every connection before you apply any power, since a wiring mistake with three wires is easier to make and harder to spot by eye than with two.
Can I use the same core I'd use for a 4:1 balun?
Yes — core selection follows the same mix and size logic either way (see the Core Selection Guide). The only practical difference is that three conductors take up more winding window space than two, so there's a slight preference for staying at FT-240 size or larger even at moderate power, simply for room to wind neatly.
Do I still need a separate 1:1 choke if I'm using a 6:1 current balun?
Not usually at the same feedpoint, since the 6:1 current balun is already providing common-mode choking there. A separate 1:1 choke at the shack entry is still a reasonable second line of defense on antennas that are especially prone to common-mode issues, the same as with a 4:1 balun.