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 perfectly matched system with proper terminations and no common-mode current. But in typical amateur setups, antennas are unbalanced and mismatched—so the coax shield is often part of the antenna.
Even a center-fed dipole, called “balanced,” becomes unbalanced due to:
- Unequal ground proximity under each leg
- Nearby objects (trees, gutters, roofs)
- Coax routing and support asymmetries
We don’t live in a NEC utopia. Perfect symmetry disappears the moment you hang a wire in the real world.
When Balance Breaks, Coax Takes Over
In ideal coax use, forward current flows on the center conductor and returns on the inside of the shield. The outside should stay quiet. But with imbalance, some of the return current spills onto the outside surface. That’s where problems start.
- TX: Return current flows on the outside of the braid, turning your feedline into a radiator.
- RX: The outside braid becomes an antenna, coupling common-mode noise from the environment into your receiver.
In Receive: Noise Pickup
The coax braid happily “hears” every switching supply, router, LED driver, or PV inverter nearby. Longer runs of unchoked coax and higher imbalance make the problem worse. The result: common-mode noise pickup that often dominates over the antenna itself.
In Transmit: Return Currents in the Shack
On TX, imbalance drives return current wherever it can close the loop—even into the shack. That’s why you see:
- RF burns
- Audio/control feedback
- RF on CAT/USB lines
- Odd SWR shifts
It’s just Kirchhoff’s Law: currents take all available paths, including unintended ones like the braid exterior.
The Role of Skin Effect
Skin effect means HF currents flow only on conductor surfaces. In coax, this separates into two independent surfaces:
- Inner wall of shield: carries legitimate differential return current.
- Outer wall of shield: should be quiet, but carries common-mode current when imbalance exists.
What’s the Fix?
The fix is strategic common-mode chokes at critical points:
- At the feedpoint — to stop the antenna from dumping current onto the coax.
- At the shack entry — to block remaining RF before it couples into equipment.
Without these, your coax is not shielded—it’s radiating and receiving.
Ladder Line: Immune by Design
Balanced line like ladder line resists this issue. Both conductors are equally exposed to fields, so common-mode cancels naturally. With proper baluns, routing, and spacing, it offers lower noise and lower loss—but requires careful implementation.
Final Words
If you think one choke is enough, think again. With verticals or random wires, both ends of coax need choking. Otherwise, the feedline itself is your noisiest antenna.
Your coax is only as good as the choke that silences it.
Mini-FAQ
- Where should I place the first choke? — At the antenna feedpoint, to stop currents before they travel the coax.
- Do I also need one at the shack? — Usually yes. It catches what the first choke missed and protects your gear.
- What ferrite mix? — For HF, use high-µ mixes (31, 75, 77) with enough turns. For upper HF/VHF, use 43 or 61.
- Will a single bead fix it? — Rarely. Proper multi-turn toroids or large multi-core chokes provide real impedance.
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