4:1 Current Balun
A 4:1 current balun does two jobs at once: it transforms the roughly 200Ω feedpoint impedance typical of an off-center-fed dipole (OCFD) or Windom-style antenna down to something a 50Ω coax line can handle, while simultaneously choking off common-mode current the way a 1:1 choke does. Get the winding topology or core sizing wrong and you can lose either function — this guide covers the bifilar winding method, core/power sizing, a turns calculator, and full troubleshooting.
Why 200Ω needs a 4:1 step-down
A dipole fed at its exact center presents roughly 73Ω. Move the feedpoint off-center (the classic 1/3-2/3 OCFD split) and the impedance at that point rises to roughly 200-300Ω depending on band, because you're now sampling a point on the antenna with a very different current-to-voltage ratio than the center. A 4:1 transformer brings that down into a range (50-75Ω) that ordinary coax and most transceivers or tuners handle comfortably, instead of forcing you to run high-SWR coax or bulky open-wire line all the way to the shack.
Current balun vs. voltage balun at this ratio
A 4:1 voltage balun (the classic W2AU-style commercial design) only forces equal-and-opposite voltage at its output terminals — it does the impedance transformation but does nothing reliable about common-mode current on the coax shield. A 4:1 CURRENT balun does the same impedance transformation while also presenting high impedance to common-mode current, because the winding is built the same way a choke is built: as a high-permeability-core inductor in the common-mode current path. For an inherently asymmetric feed like an OCFD, where unequal leg currents make common-mode current a near-certainty, the current-type design is the only one that actually works well in practice. See the Voltage vs. Current Balun guide for the full comparison.
How the bifilar winding creates the 4:1 ratio
The standard homebrew approach winds two identical, parallel ("bifilar") wires through the core together — this keeps the two windings tightly coupled, which is what makes the transformer action work correctly across a wide bandwidth. The unbalanced (coax) side connects across one winding alone; the balanced (antenna) side connects across both windings in series. Since impedance scales with the square of the turns ratio, and the antenna side sees twice the turns of the coax side, you get (2/1)² = 4 — a 4:1 impedance step between the two sides, in either direction.
Sizing for power
Because the winding does double duty (impedance transformation plus choking), it needs to handle real RF voltage without saturating — see the Core Selection Guide for the underlying flux density math. A single FT-240-43 core with 3 bifilar turns is a reasonable QRP/low-power (100-250W) starting point; stacking two FT-240-43 cores and increasing to around 9 bifilar turns (a well-tested combination used on full-power OCFD builds) is the standard choice for running at the legal power limit, especially into the elevated SWR the balun sees before final tuning.
| Core Configuration | Bifilar Turns | Approx. Winding Inductance | Recommended Power |
|---|---|---|---|
| FT-82-43 (single) | 5 | ~14 µH | Up to 50W (QRP/portable) |
| FT-140-43 (single) | 4 | ~14 µH | Up to 100W |
| FT-240-43 (single) | 3 | ~10 µH | 100–250W |
| FT-240-43 (stacked x2) | 9 | High margin | Full legal limit (1.5kW) |
4:1 Balun Turns Calculator
Materials for a bifilar-wound 4:1 current balun
A 4:1 current balun wound with 3 bifilar turns on a single FT-240-43 core, mounted in a weatherproof enclosure with SO-239 and terminal studs.
Building a 4:1 Current Balun
Budget 1-2 hours including enclosure assembly and testing. Build and verify the balun completely before connecting it to the antenna.
Prepare and wind the bifilar pair
Cut two equal lengths of wire, leaving enough extra at both ends for connections. Lay them side by side (or twist them loosely, about 1 twist per inch, for slightly tighter coupling) and wind them together through the core for the target turn count, keeping the pair oriented the same way on every turn. Mark one wire's ends with colored tape before you start winding — once both wires are through the core, it becomes easy to lose track of which is which.
Connect the transformer topology
Connect the coax center conductor to the start of winding A. Connect the coax shield/ground to the junction between the end of winding A and the start of winding B — this junction is the electrical center point that establishes the 4:1 relationship. The two antenna (balanced-side) terminals connect across the full series combination: one terminal at the start of winding A (the coax-center node) and the other at the end of winding B.
Test with a dummy load before installing
Connect a non-inductive resistor across the antenna terminals matching your expected feedpoint impedance (200Ω is standard for testing an OCFD-style 4:1 balun) and sweep SWR at the coax input with a NanoVNA. A correctly wound balun shows SWR under about 1.5:1 to 1.8:1 across your intended HF range when loaded this way — a much higher or wildly varying reading points to a winding or connection error, not a bad antenna.
Weatherproof and install at the feedpoint
Mount the tested balun in its enclosure with sealed cable entries for the coax and a drip loop below the connector. Support the enclosure's full weight independently of the terminal connections — a stainless U-bolt or integrated mounting loop, not the antenna wire itself, should bear the mechanical load.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR is high on all bands, even with correct antenna dimensions | Winding topology error — windings swapped, or shield connected to the wrong junction | Test the balun alone with a 200Ω dummy resistor across the antenna terminals; a correctly wound balun shows SWR under 1.8:1 into that load | Re-check winding connections against the topology described above; re-wind if the winding sense or junction is wrong |
| SWR looks fine but RF-in-the-shack symptoms persist | Balun is transforming impedance correctly but not choking common-mode current — often means a voltage-type balun was used by mistake | Confirm the balun is genuinely a current-type winding on a ferrite core, not an autotransformer-only voltage balun design | Rebuild as a proper bifilar current balun, or add a separate 1:1 current choke at the shack entry as a second line of defense |
| Core runs hot or cracks under power | Core undersized for actual power and SWR seen at the balun | Recalculate flux density per the Core Selection Guide using your actual RF voltage at the balun, not just rated transmitter power | Step up to a larger core or add a second stacked core, especially if running near the legal power limit |
| Antenna works well on some bands but not others | Feedpoint impedance on the problem band falls outside what a clean 4:1 step-down can bring near 50Ω | Check whether the problem band's feedpoint impedance (from antenna theory/modeling) is far from the ~200Ω the 4:1 ratio assumes | Use an antenna tuner on the affected band, or consider whether a 6:1 balun would suit that specific band/antenna length better |
Why does an OCFD need a 4:1 balun instead of a 1:1?
Because the off-center feedpoint impedance (roughly 200-300Ω depending on band) is far from the 50Ω a 1:1 balun would pass through unchanged. A 1:1 balun on an OCFD would leave you with a persistently high SWR that no amount of common-mode choking fixes, since 1:1 does nothing about the impedance mismatch itself.
Can I use a commercial 4:1 balun from an antenna kit?
Only if it's specifically labeled as a current balun (sometimes called a "choke balun"). Many inexpensive commercial "4:1 baluns" are actually voltage baluns (the classic W2AU-style design) — they handle the impedance transformation but do not reliably suppress common-mode current, which matters a great deal on an inherently asymmetric feed like an OCFD.
Do I need two stacked cores, or is one enough?
One FT-240-43 core is adequate for up to roughly 100-250W with a reasonable match. Stack two cores (winding through both together) if you're running toward the legal power limit, or if you expect the balun to regularly see elevated SWR before your antenna is fully tuned — the extra core cross-section reduces flux density and heating for the same power.
How do I know if my finished balun is actually working correctly?
Test it before installing: connect a 200Ω non-inductive resistor across the antenna terminals and sweep SWR at the coax input with a NanoVNA. SWR under about 1.5:1-1.8:1 across your HF range confirms the transformer is working; if you see high or erratic SWR into that known-good resistive load, the problem is in the balun, not the antenna.
My SWR is good on one band and terrible on the rest — is that the balun?
This specific pattern (good on one band, high everywhere else) is the classic symptom of the two antenna wire legs being connected to the wrong terminals, not a balun defect — swap the two antenna-side connections and re-test before assuming the balun itself is faulty.