Noise Coupled ≠ Noise Radiated

Updated: January 2026 — technical review and terminology alignment.

Why “pickup is differential” while “coupling is common-mode” — and why a coax can go bananas once imbalance lets RF out of the pipe.

The one-sentence picture

Stray return current is like RF escaping the pipe… and that same exterior path, in receive, becomes a conduit for common-mode noise coupling.

That single idea ties together why a coax normally neither radiates nor picks up much, why imbalance causes antenna pattern distortion, why common-mode chokes sometimes feel magical (and sometimes useless), and why fixing the antenna alone is never enough in a modern noise-dense environment.

Two fundamentally different noise paths

Door A — Radiated noise (differential pickup)

Noise leaves a device as an electromagnetic field, propagates through space, and is received by the antenna like any other signal. The resulting voltage appears between the antenna terminals and is therefore differential-mode.

• The antenna alone is sufficient to receive it
• The feedline does not need to be part of the antenna
• A choke on the coax will not remove it

Door B — Coupled noise (common-mode coupling)

Noise couples onto station conductors — most notably the outside of the coax shield — through capacitive or inductive coupling and messy return paths.

• The coupling manifests as common-mode pickup
• The feedline exterior becomes a receiving structure
• Chokes can dramatically reduce it

Both doors can exist simultaneously, but the cure is not the same.

Differential vs common-mode — in plain language

A coax supports two current systems at the same time:

Differential mode (wanted)

• Current flows on the center conductor
• Equal and opposite return current flows on the inside of the shield
• Fields are largely confined inside the coax (TEM mode)

Common-mode (unwanted)

• Current flows on conductors with respect to their surroundings
• Returns through shack wiring, equipment, mains earth, and random metal
• Radiates and receives efficiently

This is the EMC intuition that matters: differential currents are geometry-dependent; common-mode currents are environment-dependent.

50 Ω Coax — Balanced at Its Design Impedance, Unbalanced When It’s Not

A 50 Ω coaxial cable is often called “unbalanced” because of its physical construction. That description is incomplete in an RF sense.

At its design impedance, a coax is electrically balanced.

• Equal and opposite currents flow on the center conductor and the inside of the shield
• Fields are confined inside the cable
• The outside of the shield carries no significant RF current

In that condition, the coax behaves as a controlled TEM transmission line and is externally RF-quiet.

Imbalance appears when the system is forced away from that condition.

• Part of the return current is diverted onto the outside of the shield
• A stray return current path is created
• That external return path is still part of a differential current system, but it now exists outside the intended TEM geometry and can couple efficiently to common-mode noise fields

At that point, RF has effectively “escaped the pipe.” The system still carries differential current, but no longer in a controlled geometry. Because that stray return path exists with respect to its surroundings, it can radiate — and in receive, act as a conduit for common-mode noise coupling.

Engineering footnote: In strict electromagnetic terms, “common-mode” describes current flowing on conductors with respect to a reference environment. A stray return current path may be differential in nature, yet still be highly susceptible to common-mode coupling because it is no longer field-confined.

For a deeper technical treatment, see:
50 Ω Coax — Balanced at Its Design Impedance, Unbalanced When It’s Not

Why stray return currents cause both TX and RX havoc

Antenna pattern distortion

Once the feedline carries RF outside the intended geometry, it becomes part of the radiating structure, altering lobes, nulls, and pattern stability.

More RF in the shack

That same current flows through the radio chassis and connected cabling, producing classic RFI symptoms.

In receive: a noise conduit

The exterior path eagerly couples to E-field noise from modern electronics.

Why chokes help — and why they don’t always

A common-mode choke impedes current that interacts with the environment. It can greatly reduce noise coupling on stray return paths, but it cannot remove a legitimate differential signal received by the antenna.

Fast diagnostic checklist

• Dummy load test — antenna path or conducted?
• Choke test — common-mode coupling or radiated pickup?
• Portable receiver walk-around
• Directionality check

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