sheath current filter: Poorly shielded antenna cables, older antenna sockets and other influences (e.g. telephone cable or socket in the vicinity under certain circumstances) can generate interference signals on the screen of a television set via the antenna input.
A sheath current filter, which is inserted between the plug of the antenna cable and the antenna socket on the television set, can help. This filters out all frequencies that do not belong to the television signal (4 – 862 MHz).
A sheath current filter is usually a cable adapter to electrically decouple devices that are connected to an antenna connection. There are also antenna connection cables with built-in sheath current filters.
With a cable connection or satellite reception system, the shielding of the cable (the sheath) often has a different ground potential than the devices in the living room. The moment you then plug the antenna cable into a device that has a Schuko plug (normal plug for the socket), you often hear a 50 Hertz-Humming from the speakers. Also, the image may be affected. A sheath current filter is used to avoid this. The price for a sheath current filter is around 5-15 euros and such a filter should be available at any good electronics store.