Common-Mode and Return Currents on Coax: Your Feedline Is Fighting You

Many hams misunderstand what coax is actually doing in their station—especially when their antenna system isn't perfectly balanced. And here’s the kicker: no antenna is ever perfectly balanced in the real world. Not even a textbook dipole.

Let’s break it down.

The Myth of Coax as a Shielded Savior

Most hams think coax “contains” RF. That’s only true in a matched system with proper terminations and no common-mode. But in most ham setups, the antenna is far from balanced—and often mismatched. That’s where things go wrong.

Even a center-fed horizontal dipole, often called a “balanced” antenna, becomes unbalanced due to:

• Ground proximity under one leg
• Asymmetrical environment (trees, gutters, roofs)
• Unequal coax routing or support structures

We don’t live in a NEC utopia. Theoretical symmetry quickly collapses when you hang a wire over your garden shed.

When Balance Breaks, Coax Takes Over

Coax is designed to carry RF energy inside the shield (differential mode): forward current on the center conductor, return current on the inner wall of the shield. When all goes well, the outside of the shield stays quiet.

With imbalance—however slight—the return current no longer stays entirely on the inside surface of the shield. It also flows along unintended paths that include the outside of the shield.

• On transmit (TX): this is still a differential-mode return current, but forced onto the braid’s exterior by the imbalance—making parts of the feedline radiate.
• On receive (RX): the outside of the shield behaves like its own antenna and picks up common-mode noise from the environment.

In Receive: It Pulls In Every Noise Source Around

On RX, the outside of the shield picks up every switching supply, router, LED lamp, PV inverter, and SMPS. The effect grows with longer, unchoked coax and greater antenna imbalance. This is pure common-mode reception.

In Transmit: Return Currents Flow the Wrong Way

On TX, the imbalance forces the differential return current to wander—often along the outside of the braid and into the shack. That’s how you get:

• RF burns
• Weird audio or control behavior
• Mystery feedback loops
• RF on CAT/USB lines

It’s just Kirchhoff’s Law: current completes its circuit by any available path—even if that path is your coax braid and your PC.

The Role of Skin Effect

Skin effect confines high-frequency currents to a conductor’s surface. In coax, this yields two distinct surfaces on the shield:

• Inner wall: carries the intended return current (normal/differential mode).
• Outer wall: should be quiet, but carries common-mode when the system is imbalanced.

What’s the Fix?

The reliable cure is strategic common-mode choking:

1. At the feedpoint — stop the antenna from dumping current onto the coax.
2. At the shack entry — prevent any remaining RF from returning into your gear.

Without these, your coax isn’t shielded—it’s exposed.

Ladder Line: Immune by Design

Balanced line like ladder line doesn’t suffer this issue the same way. Its conductors are symmetric and equally exposed to fields, so common-mode cancels naturally. The result can be cleaner RX and lower loss—if you implement it correctly (proper balun, routing, and spacing).

Final Words

If you're using coax and think a single choke is enough, think again. In a mismatched or unbalanced system—especially with random wires or verticals—both ends of the coax benefit from choking. Otherwise, your feedline becomes your biggest liability.

Your coax is only as good as the choke that silences it.

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