Each current floating on the outside of a coax is not common mode

Why return current, unbalanced feed, and common mode are not the same

Many radio amateurs believe that any RF current measured on the outside of a coaxial cable is "common mode." While this is a tempting simplification, it’s technically incorrect — and can lead to poor troubleshooting, misapplied chokes, and incorrect assumptions about RFI sources.

Let’s clarify the difference.

Differential current and return paths

In a properly working RF system, your antenna feedline carries a differential signal:

  • The forward current flows from the transmitter to the antenna along the center conductor.
  • The return current flows back along the inside of the coax shield.

Together, they form a closed differential loop. The skin effect ensures that these currents remain separated and confined to their respective surfaces.

What happens in unbalanced feeds (like end-feds)

When you feed an antenna asymmetrically — such as an end-fed half-wave (EFHW) or off-center-fed dipole — you no longer have a natural current balance. There is no matching counterpoise or opposing conductor to close the loop symmetrically. As a result:

  • Part of the differential return current seeks an alternate return path.
  • This path often includes the outside of the coax shield, your station ground, rack, tuner, or other conductive structures.
  • But this is still differential current — just flowing along an unintended return path.

This current on the outside of the coax isn’t inherently common mode. It’s simply displaced differential return current due to an incomplete or poorly defined antenna current loop.

Note: Even when antenna imbalance causes current to flow on the outside of the coax shield, that current is still part of the differential system. It flows in the opposite direction to the center conductor current, completing the transmit loop. It is therefore not common mode current, even if it appears in unexpected places.

Why it still flows in the opposite direction

At first glance, it might seem counterintuitive that current on the outside of the coax shield would flow in the opposite direction of the center conductor and still be part of the differential loop. But RF systems obey Kirchhoff's Current Law: the current that leaves a source must return to it.

In a well-balanced system:

  • Forward current flows out via the center conductor.
  • Return current flows back via the inside of the shield, 180° out of phase.

In an unbalanced system (like an end-fed):

  • There is no proper counterpoise.
  • Return current finds an alternate path — often over the outside of the coax shield.

However, this current is still returning to the transmitter. It is not an independent signal, and it exists only because of the forward current. Therefore, it still flows in the opposite direction, fulfilling the same differential loop — albeit in an unintended way.

This is not common mode: in true common mode, current flows in the same direction on both conductors or surfaces, with no differential loop. That is a fundamentally different phenomenon.

What is common mode current then?

True common mode current occurs when the same current flows in the same direction on both the center conductor and shield. This is typically the result of:

  • External electric field coupling (e.g., near strong sources or long cables)
  • Mode conversion (from poor shielding or antenna imbalance)
  • Shared grounds or coupling via power lines

Unlike differential current, common mode current does not require a matched return path. It radiates independently and is a typical cause of EMI, RFI, hum loops, and pickup of environmental noise.

Why it matters

Mislabeling all shield current as "common mode" leads to:

  • Overuse or misplacement of chokes
  • Undervaluing the role of counterpoise systems
  • Misunderstanding why end-fed antennas cause RFI
  • Incomplete EMC and station design

Rule of thumb

If there’s a current loop with a driving source and a return — even if the return is "messy" — it’s differential.
If there’s current flowing equally on both conductors in the same direction, that’s common mode.

Both can radiate. Both can be unwanted. But they are not the same, and fixing one may not fix the other.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.