What are common-mode currents on antenna feed lines?

Common-mode current is current that flows on the outside surface of a coaxial cable shield (or equally on both wires of a balanced line), as opposed to the differential currents inside the line. It arises when a balanced antenna is fed directly with coax, when the ground plane is small, or when the antenna is asymmetric, and it makes the feed line radiate, distorting patterns, coupling noise and making measurements depend on cable routing.^[1][2]

Why does common-mode current appear?

Skin effect separates the inner and outer surfaces of the shield. At a dipole feed, the inner shield current divides between the antenna arm and the outer shield surface according to their impedances, so part of the current flows back down the outside of the cable. Small ground planes in embedded devices have the same effect: the cable becomes the counterpoise.^[1]

How do you suppress it?

• Use a current balun or choke at the feed point: coax wound on ferrite, ferrite sleeves, or an air-wound coil.
• Use λ/4 sleeve baluns for narrowband antennas.
• Add additional chokes along the cable for long runs.
• Use ferrite-loaded or optical links in antenna measurements.^[2][3]

Why does it matter for measurement?

A small DUT measured with a coax cable may show resonances and efficiencies that belong to the cable. Chamber measurements of small devices therefore use ferrite-loaded cables, baluns at the feed, battery-powered radios or fiber-optic links.^[3]

Frequently asked questions

See also

• Baluns
• Feed lines
• Measurement overview
• Ground-plane effects

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

1. “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
2. “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.
3. “IEEE Recommended Practice for Antenna Measurements (IEEE Std 149-2021)”. IEEE Antennas and Propagation Society. 2022.