Complex sentence: Usually mast antennas are series-excited (base driven); the feedline is attached to the mast at the base, so the base of the antenna is at high electrical potential and must be supported on a ceramic insulator to insulate it from the ground.
Split into simpler sentences: Usually mast antennas are series-excited (base driven); the feedline is attached to the mast at the base. The base of the antenna is at high electrical potential and must be supported on a ceramic insulator to isolate it from the ground.