When Open-Wire Feedline Starts to Radiate

and Why It’s Not Just a “Coax Problem”

Many hams believe that only coaxial cable can radiate — because “it’s unbalanced.” But that’s not true. Both open-wire lines and coaxial lines can radiate when balance or symmetry is lost. Geometry alone doesn’t make one immune. What matters is the current balance between conductors, not whether they’re twisted, shielded, or floating in air.

Differential vs “Common-Mode” Current

Every feedline supports two possible ways current can flow:

• Differential mode — equal and opposite currents on each conductor. Their electromagnetic fields cancel in the far field, so the line carries energy efficiently but does not radiate.
• Imbalance (often mislabeled “common mode”) — occurs when those equal-and-opposite currents no longer match.
  • In receive, an external reference exists — the environment or ground — so a true common-mode current can flow relative to that reference. It picks up or radiates noise just like an antenna.
  • In transmit, there is no third conductor or ground reference completing a separate common-mode path. The imbalance simply redistributes according to Kirchhoff’s current law: one side carries slightly more, the other slightly less, and the mismatch current flows along nearby conductors (coax shield, mast, open-wire leg, etc.). The resulting external field is radiation caused by asymmetry — not by any “third wire.”

In both cases, what radiates is the unbalanced portion of current that lacks an equal and opposite partner to cancel its field — not the feedline geometry itself.

Where Imbalance Comes From

Even in a perfect doublet, real-world imbalance creeps in through:

• Unequal proximity to ground or nearby objects (trees, tower legs, roof edges).
• Different wire heights or twists causing unequal coupling to ground.
• Feedpoint asymmetry — e.g., the antenna isn’t centered, or each leg sees different reactance.
• Asymmetric coupling into the tuner or balun.

Once the fields on each conductor differ, their cancellation fails and the line itself starts to radiate. The “balanced” open-wire line becomes part of the antenna system.

Radiation Doesn’t Care About Geometry

This is where the “only coax radiates” myth collapses. Coax is called “unbalanced” only because one conductor (the shield) is often grounded, not because its currents magically lose symmetry. If you drive it correctly — equal and opposite currents inside — the coax will not radiate. If you drive open-wire incorrectly — with imbalance — it will radiate. The geometry merely hides or exposes the imbalance; it doesn’t cause or prevent it.

In other words: radiation follows current imbalance, not cable type.

When Does Open-Wire Actually Radiate?

We can define the point where open-wire line starts radiating as when the imbalance current exceeds a small fraction of the differential current. In practice, if the unbalanced component is above about –20 dB (1/10 of the differential level), measurable feedline radiation begins. At –10 dB it becomes dominant — your line is now part of the antenna, altering the pattern and feedpoint impedance.

Typical causes include:

• Feeding a non-symmetric antenna (e.g., off-center fed doublet) directly with open-wire line.
• Running the line very close to ground, metal gutters, or vertical supports.
• Passing the line through a window panel with one side against the frame.
• Using a tuner without a proper current balun at its balanced port.

Measuring and Mitigating It

There are a few ways to detect when your open-wire feedline is radiating:

• Touching or moving the feedline changes the SWR or noise level — a strong hint of imbalance current.
• Common-mode chokes (or 1:1 current baluns) reduce noise pickup or RFI in the shack — confirmation that external current existed.
• Using a clamp current probe, you can measure equal and opposite currents on both conductors. Any difference between them is radiated current.

Mitigation strategies include:

• Keep the feedline well away from conductive structures.
• Run open-wire lines at right angles to the antenna for at least 0.1 λ before turning.
• Use a current balun at the transition to unbalanced gear.
• Maintain symmetry in antenna length and height.

Coax, Open-Wire, Twin-Lead — Same Physics

All transmission lines obey Maxwell’s laws, not folklore. The outer surface of a coax shield can carry unbalanced current just like both conductors of an open-wire line can. Inside the coax, the differential fields cancel; outside, any imbalance produces radiation. In open-wire, both conductors are “outside,” so it’s easier to disturb balance — but the principle is identical.

The line that radiates least is the one carrying only differential current, regardless of shape or shield.

Takeaway

Open-wire doesn’t magically avoid radiation, and coax doesn’t automatically cause it. Both behave perfectly well when used within their intended balance conditions. Radiation appears only when the feed system becomes part of the antenna — through imbalance, not geometry.

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