Sheath current

A standing wave is a form of electromagnetic wave on lines. On a conductor it runs as along the outer sheath of a coaxial line. The return conductor is the mass system surrounding the outer conductor. This may for example consist of a spatially near or distant ground.

Coat waves may also be caused by induction of the actual transmission signal to the outside of the Koaxschirms. Here, the standing wave trap must, however, not be attached at the beginning or end of the line, but in the current loop of the standing wave. This barrier is dependent on frequency and therefore very problematic in multi-band antennas, particularly when the antenna line is implemented parallel to the radiator.

Standing waves can significantly reduce the efficiency and interfere with electronic equipment in the vicinity due to the standing wave associated with the unwanted transmission of electromagnetic radio waves in transmission facilities. In addition, lead sheath waves that occur at the ends of a coaxial cable by a difference in ground potential to common mode signals that are superimposed on the useful signal and noise voltage. Standing waves can be the cause of ground loops.

Measures to reduce standing waves

High-frequency standing waves can be by means of a standing wave filter ( also standing wave absorbers or inhibit), which is applied to a coaxial line in or near the device, attenuate or prevent. In the simplest case, this is a ferrite core; it comprises the coaxial cable inner conductor and outer conductor and acts as a current-compensated choke or common mode choke. At the same time, a ferrite core transformer acts, so that a useful signal is supported as a push-pull signal. To increase the inductance compared to the unwanted common mode signal component, the cable may be performed more than once through the core.

In addition, often capacitive sheath current filters. More information can be found in the article sheath current filter.

With a balun to standing waves can be avoided if a balanced line to an unbalanced line such as a coaxial cable is connected. Without the use of baluns incur standing waves on the unbalanced line. One use case is the connection of a symmetric dipole antenna with a coaxial line.

Sheath wave barriers in amateur radio

Koaxialkabelgespeiste antennas or poor grounding high-frequency currents often lead to standing waves. A simple solution for the suppression of these waves are about 10 turns of coax around an iron ring with maximum permeability.