English: Transmission line emerging from the transmitter building of Sendeanlage Bisamberg, a medium wave AM broadcasting station in Bisamberg near Vienna, Austria. It feeds radio power to the north antenna mast. This type of transmission line is called a "cage line". It consists of a small circular bundle of parallel wires carrying the RF current, surrounded by a larger circle of grounded wires, which function as a "shield" to prevent the power from radiating off the inner wires as radio waves. It functions similarly to a large version of a coaxial cable.