Also known as Big Clamp On. Very useful for AC/DC power, coax cable, etc. Use multiple turns for best results or multiple chokes in series (recommended for large amperage power cables). Always put +/- dc cables through the same ferrite filter so their fields cancel and the ferrite suppresses the common mode current on each cable.
These ferrites also work well for coax feed line chokes. Use 3 turns of RG-213 size coax thru center and put several ferrite in series each with 3 turns to make an excellent 80-10 meter high power feed line choke.