Why “Common Mode” Is the Most Abused Term in Ham Radio

Many hams call any current on a coax shield “common-mode current.” In scientific EMC and transmission-line theory, that’s not what common-mode means. The term has been over-simplified and stretched until it obscures the physics and leads to the wrong fixes.

What Common-Mode Really Means

In EMC and transmission-line theory, common-mode current is current that flows in the same direction and with equal magnitude on all conductors of a system, referenced to a common point (often chassis or earth). Textbook definitions:

• Clayton R. Paul, Introduction to Electromagnetic Compatibility, 2nd ed., Wiley (2006): “Common-mode currents are those flowing in the same direction on all conductors of a cable, relative to a common reference.”
• Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley (2009): “Common-mode voltage is the average potential of all conductors relative to a common ground. Common-mode current is the sum of conductor currents flowing in the same direction with respect to that reference.”
• ITU-T K.117 (2016): “Common-mode (CM): A signal or disturbance appearing equally and in phase on two or more conductors with respect to a reference point.”

In short: genuine common-mode requires a defined external reference.

Why This Breaks Down on Transmit

On transmit, the antenna system is a closed differential circuit. RF current leaves the source, flows through one path, and returns through the other, obeying Kirchhoff’s law. There is no third reference for the intended signal itself.

So the feedline current that worries us on TX is not “common-mode” in the strict EMC sense — it is differential return current imbalance. Because of asymmetry (geometry, height, nearby structures, ground coupling), part of the return finds the “easiest” path on the outside of a coax shield or along nearby conductors. The current on the outside of coax is thus better described as an unbalanced differential return, not a true common-mode pair.

We use chokes and baluns to confine or block these stray return paths and restore symmetry — not to “cancel common-mode” on a balanced, floating TX circuit.

Receive vs. Transmit Reality

Receive: The receiver and cable typically reference the station chassis/earth. Ambient electric fields couple equally onto both conductors → a textbook common-mode noise situation. A good choke reduces that noise current.

Transmit: The same choke also helps, but now by impeding imbalance currents that ride on unintended paths. The term “common-mode” is a misnomer here; the unwanted current exists, but its origin is differential imbalance, not an in-phase, equal-magnitude pair referenced to ground.

Why the Term Got Abused

Both problems (shield radiation on TX and noise pickup on RX) often improve with the same remedy: a choke. Over time, “common-mode” became shorthand for “any unwanted current on the outside of a cable.” That simplification helps teaching but blurs the physics and leads to design myths.

Field Perspective

• Differential mode: equal and opposite conductor currents; fields tend to cancel locally; radiation is intentional from the antenna element.
• Common mode (strict): equal and in-phase currents relative to a reference; fields reinforce; cables and structures radiate efficiently.

Most “feedline radiation” on TX is due to imbalance converting part of the differential system into a net external field on the cable surface.

Ladder Line and “Common Mode” Reality

Ladder line has the same potential issues as coax: imbalance currents on transmit and common-mode pickup on receive. The difference is structural. Coax offers three conductive surfaces (inner conductor, shield inner surface, shield outer surface). Open-wire line has only two conductors. When imbalance occurs, coax return can split between inner shield surface and outer surfaces/adjacent structures; on open-wire, the return must redistribute between the two wires themselves. The mechanisms are the same — only the geometry and containment differ.

Balanced line does not magically prevent these effects; it simply makes them easier to manage when the entire system (antenna, feedline routing, environment) remains symmetric.

So What Should We Call It?

Use precise language:

• Imbalance current on the feedline (transmit case).
• Common-mode noise pickup (receive case).

Both are mitigated by chokes, but they arise from different physics. That distinction guides better fixes: improve feedpoint symmetry and return paths on TX; improve shielding, bonding, and choke impedance at RX entry points.

The Takeaway

Before blaming “common-mode,” ask: Is it truly common-mode referenced to ground, or is it differential imbalance seeking a return path? Precision in terminology drives precision in design — and fewer myths.

Tip: Place the primary choke as close as practical to the feedpoint. Add secondary chokes where the line approaches conductive masts, bends, or entry points to limit re-induced currents.

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