Start here: Differential-mode current flows out on one conductor of a transmission line and returns with equal magnitude on the other; it is the intended current that carries power to the load.
Why it matters
On coax it flows on the center conductor and the inner surface of the shield. Equal and opposite fields largely cancel outside an ideal line.
A practical example
A clamp around the entire coax should see little ideal differential current; a reading there usually points to common-mode current on the exterior.
Use it well
Keep differential and common-mode measurements separate when diagnosing feed-line radiation.
Operating boundary
Keep the measurement reference plane, current program rules, land access, spectrum authorization, and exact packed configuration with the decision. A connector, meter reading, or program name alone does not establish readiness.
