A sheath current is a form of electromagnetic wave in wires. Sheath currents can run along the outer sheath of a coaxial cable

Coaxial cable

Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...

. This can be caused by a geographically proximate or remote ground potential.

Sheath currents may lower the efficiency of transmission and can interfere with nearby electronic devices. In addition, sheath currents caused by differences in ground potential at the ends of a coaxial cable to common mode signals that are superimposed on the useful signal as a noise voltage. Sheath currents can be caused by ground loops

Ground loop (electricity)

In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...

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Countermeasures against sheath currents

High-frequency signal lines may attenuate or prevent sheath currents using a sheath current filter which is applied to a coaxial cable in or near the device. In the simplest case, this is a ferrite bead

Ferrite bead

A ferrite bead is a passive electric component used to suppress high frequency noise in electronic circuits. It is a specific type of electronic choke. Ferrite beads employ the mechanism of high dissipation of high frequency currents in a ferrite to build high frequency noise suppression devices...

, it includes the coaxial inner and outer conductor and acts as a common-mode

Common-mode signal

Common-mode signal is the component of an analog signal which is present with one sign on all considered conclusions. In electronics where the signal is transferred with differential voltage use, the common-mode signal is called a half-sum of voltages:...

 choke

Choke (electronics)

A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...

. At the same time, a ferrite bead has a transformer effect, so that a useful signal is confirmed as differential-mode. To increase the inductance compared to the unwanted common mode signal component, the cable can also be repeatedly passed through or wound around the bead.

In addition, are often used capacitive coupling filter. More information is available in the article sheath current filter

Sheath current filter

Sheath current filters are electronic components that can prevent noise signals travelling in the sheath of sheathed cables, which can cause interference...

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With a balun

Balun

A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...

, sheath currents can be avoided when a balanced line is connected to an unbalanced line. Without the use of baluns, sheath currents will occur on the unbalanced line. A use for this is the combination of a symmetric dipole antenna

Dipole antenna

A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...

with a coaxial line.