The Common Mode Conundrum: When Physics Meets Folklore

By a weary but persistent electron herder

Once upon a time in the mythical land of Hamistan, a great tale was passed down through generations of operators, etched into the minds of Elmers and newcomers alike:

"If current flows on the outside of your coax, it must be common mode! And beware—your station shall be cursed with RFI and poor SWR forever!"

And so it was believed.

You see, in Hamistan, no tale is more fearsome than the story of the Evil Outside Current—that spectral flow of electrons said to possess coaxial cables, causing tuners to howl and baluns to melt in despair. "It’s common mode!" they cried. "It’s evil! It disobeys all laws of physics and phase—probably funded by big ferrite!"

But here's the rub.

Despite the legend, those pesky electrons are still following Kirchhoff’s Current Law—that dry, boring law from the Land of Engineering that states:

“All currents that go out must come back.”

Yes, even the ones shimmying on the outside of the coax are simply part of the differential signal, coming back home like loyal little salmon. They didn’t take a detour through Satan’s feedline. They didn’t change phase like it’s some kind of Las Vegas magic act. They’re just... electrons. Doing their job.

But try telling this to the village council of Hamistan.

"No!" they shout, pounding their copy of the ARRL Handbook. "If it’s on the outside, it’s common mode! It must be banished with ferrite! Lots of ferrite!"

And so, they wrap their cables like mummies, chant SWR incantations, and post grainy photos of their shack on QRZ.

Meanwhile, the return current—oblivious to the drama—is still flowing in the opposite direction to the forward current, happily closing the loop, perfectly differential.

Not every current on the outside is a villain. Sometimes, it’s just the unpaid intern of physics — trudging back home, obeying Kirchhoff like it's still under contract. Even when there’s imbalance at the load, this loyal little current remains part of the differential pair. It didn’t join a cult, change hats, or declare itself common mode just because it’s walking on the outside. No, it’s still returning what the center conductor sent — just taking the scenic route along the coax shield. If that sounds spooky, blame Kirchhoff, not dark magic.

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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.