Coaxial Cable: The Myth of Being "Unbalanced"
One of the most persistent misconceptions in ham radio is that coaxial cable is inherently “unbalanced.” While it looks asymmetrical, that’s geometry — not necessarily how currents behave. Let’s unpack this properly.
Geometry vs. Operation
Coax is called "unbalanced" because its geometry is asymmetrical: one conductor is inside, the other forms a surrounding shield. But geometry does not dictate electrical balance. In a properly terminated system, the center conductor and the inside of the shield carry equal and opposite currents. The external field cancels, and the cable neither radiates nor picks up RF. That is electromagnetically balanced behavior.
So Why the Confusion?
The myth comes from equating lack of symmetry with lack of balance. In reality:
- Coax feeding a truly balanced load (via a balun or current choke) behaves as a balanced transmission line.
- Problems arise when the load or feedpoint is not balanced. Then return currents can’t stay confined to the inside of the shield.
- The coax shield begins to act as part of the antenna, radiating and picking up noise. This is often mislabeled “common-mode current.”
In truth, what you’re seeing is imbalance-driven return current. True common-mode current is when both conductors carry current in the same direction with respect to ground. In practice, imbalance and common-mode are intertwined — imbalance opens the path for common-mode to flow.
- Ideal (balanced): +I flows on center conductor, –I flows on shield interior. Net external field = zero. Coax is quiet.
- With imbalance: +I flows on center, –Ishield is smaller. The remainder flows on the outside of the shield. Coax radiates or picks up RFI.
- True common-mode: both conductors carry in-phase current relative to ground. Entire coax radiates like a single wire.
This is why a current balun or choke is critical: it forces equal and opposite currents inside the coax, blocking the imbalance from spilling outside.
Balanced vs. Symmetric
Open-wire line is physically symmetric and typically operated in balanced mode. Coax is physically asymmetric but can be electrically balanced if currents are properly managed. Balance is about current conditions, not cable geometry.
What You Really Need to Know
- Coax is not inherently “unbalanced.” It’s a shielded structure that can carry balanced currents.
- It becomes unbalanced when the system introduces asymmetry — antennas without proper counterpoise, poor feedpoint balance, or missing chokes.
- A current balun or line isolator at the feedpoint is the most effective way to stop shield current and preserve proper coax behavior.
Final Thought
Calling coax “unbalanced” without context is misleading. It’s balanced when used correctly. The shield only radiates if imbalance forces current onto it. Always think in terms of current return paths, not just geometry.
Balance is a current condition — not a cable type.
Mini-FAQ
- Is coax really unbalanced? — No. Coax is electromagnetically balanced when currents are equal and opposite on its conductors.
- Why does my coax radiate? — Because of feedpoint or system imbalance, forcing return current onto the shield exterior.
- Will a choke fix it? — Yes. A current balun or isolator forces balance and blocks shield currents.
- Is symmetry the same as balance? — No. Coax lacks symmetry but can operate in balance; open-wire has symmetry but can be unbalanced if misused.
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