English: Contractors working on an insulator on an antenna feed bus tower of the VLF transmitter at the US Navy's Jim Creek Naval Radio Station, in Oso, Washington State. This station transmits orders to US Navy submarines in the Pacific fleet at a frequency of 24.8 kHz with an output power of 1.2 megawatts, one of the most powerful radio transmitters in the world. The tower is one of 10 which support two feed buses which extend along the valley floor from the transmitter building, feeding power to an enormous overhead antenna consisting of 10 cables stretching 1.5 miles between the ridges on the opposite sides of the valley. From the feedpoint (shown here) at each feed bus tower a cable extends up to attach to the midpoint of one of the valleyspan cables. In operation the cables carry voltages of 200 kilovolts, requiring the long 15 ft. insulators shown. The donut shapes are corona rings to prevent the high voltage electricity leaking off the antenna into the air through corona discharge.