Bifilar vs. Coax: Which Winding Is Best for Chokes?
Bifilar windings — two wires laid side-by-side on a core — often appear in RF choke designs. They look neat, balanced, and symmetrical. But are they better than coax windings?
The answer depends entirely on what you're feeding.
Coax Feedline? Stick With Coax Windings
If you're choking common-mode current on coaxial cable, using coax itself as the winding medium performs best — by far.
When you wind coax onto a ferrite core:
- The signal currents (on the center conductor and the inside of the shield) remain confined inside the cable.
- The common-mode and imbalance return currents (on the outside of the shield) loop through the ferrite and get attenuated.
- The result: high choking impedance, low loss, clean response, and excellent repeatability.
Why not bifilar here? Because it’s not truly coax anymore. Bifilar designs introduce capacitive coupling, unbalanced magnetic fields, and layout dependencies that can degrade choking performance — especially at higher HF.
Balanced Line or Coax-to-Balanced? Use Bifilar
Where bifilar windings shine is in balanced-to-balanced or unbalanced-to-balanced scenarios:
- Ladder line
- Open wire
- Twin-lead (300–600Ω)
- Coax-to-ladder-line transitions
- Unbalanced tuners feeding balanced lines
Here, bifilar windings enforce equal current paths and symmetry, keeping balanced lines clean from stray or common-mode currents.
Even when using an unbalanced tuner to feed ladder line, you still need a bifilar 1:1 choke — right at the tuner output. It prevents the tuner and its wiring from radiating and ensures the ladder line behaves as intended.
⚠️ Common Mistake: Mirrored Bifilar on Both Sides
Some builders try to be clever and use mirrored bifilar windings on both sides of a core. The idea is to keep symmetry — but in practice this sucks common-mode energy into the system and creates imbalance instead of preventing it. The result: degraded choking impedance and more RF riding the feedline.
Rule of thumb: use bifilar windings only where balanced currents are required, and keep coax windings strictly for coax suppression.
⚠️ Don't Cross the Streams
Mixing the wrong winding with the wrong line yields degraded performance:
- Bifilar on coax: unpredictable choking, capacitive imbalance, poor high-frequency response.
- Coax winding on ladder line: not balanced, doesn't enforce current symmetry.
- Omitting a choke when transitioning from coax to balanced line: welcome to common-mode hell.
Summary: Choose Based on Line Type and Function
| Winding Type | Best Use Case | Coax CM Suppression | Ladder Line Balance | Coax to Balanced Line |
|---|---|---|---|---|
| Bifilar Wire | Balanced transmission lines or transitions | ⚠️ Inconsistent | ✅ Excellent | ✅ Excellent |
| Coax Cable | Common-mode suppression on coax feedlines | ✅ Excellent | ❌ Not suitable | ⚠️ Poor balance |
Final Word
- Use bifilar windings when dealing with balanced lines, or transitioning from coax to ladder line.
- Use coax windings when suppressing common-mode on coax.
- Avoid mirrored bifilar chokes — they make imbalance worse, not better.
Same core. Same number of turns. Different job, different tool.
Mini-FAQ
- When should I use bifilar windings? — For ladder line, open-wire, or coax-to-balanced transitions where symmetry matters.
- When should I use coax windings? — For suppressing common-mode currents on coaxial feedlines.
- Why avoid mirrored bifilar? — It couples common-mode back into the system, creating the imbalance it should prevent.
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