Jacket wave
A standing wave is a form of electromagnetic wave on lines. On a conductor, for example, it runs along the outer sheath of a coaxial line . The return conductor is formed by the ground system surrounding the outer conductor. This can for example consist of a spatially close or distant earth potential.
Sheath waves can also arise through induction of the actual transmission signal on the outside of the coaxial screen. Here, however, the standing wave barrier does not have to be attached at the beginning or at the end of the line, but in the flow belly of the standing wave. This lock is frequency-dependent and therefore very problematic with multi-band antennas, especially when the antenna feed line is run parallel to the radiator.
Standing waves can reduce the efficiency of transmitter systems and, because of the unwanted emission of electromagnetic radio waves associated with the standing wave, interfere with nearby electronic devices. In addition, standing waves, which are caused by differences in the ground potential at the ends of a coaxial cable, lead to common-mode signals that are superimposed on the useful signal as interference voltage. Standing waves can cause hum loops .
Measures to reduce standing waves
High-frequency standing waves can be attenuated or prevented by means of a standing wave filter (also known as a standing wave absorber or barrier), which is applied to a coaxial line in or near the device. In the simplest case this is a ferrite core ; In the case of coaxial cables, it comprises inner and outer conductors and acts as a current-compensated choke or common-mode choke. At the same time, a ferrite core acts as a transformer , 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 can also be fed through the core several times.
In addition, capacitive sheath current filters are often used. More information can be found in the article Mantelstromfilter .
With a balun , standing waves can be avoided if a symmetrical line is connected to an asymmetrical line, e.g. B. a coaxial cable is connected. Without the use of the balun, standing waves would arise on the unbalanced line. One application for this is the connection of a symmetrical dipole antenna with a coaxial line.
Standing wave barriers in amateur radio
Antennas fed by coaxial cables or poor grounding of high-frequency currents often lead to sheath waves. A simple solution to suppress these waves is about 10 turns of coaxial cable around an iron ring with the highest possible permeability .
Individual evidence
- ↑ The craft bunker: Balun made of RG58 and Amidon core .