Coaxial cable
Coaxial cable is probably the most widely used type of feedline, used for frequencies below the microwave (SHF) range. It consists of a wire center conductor and a braided or solid metallic "shield" conductor, usually copper or aluminum surrounding it. The center conductor is separated from the outer shield by a dielectric, usually plastic foam, to keep the separation between the two conductors precisely constant. The shield is covered with an outer plastic insulation jacket. In "hard coax" cable, used for high power transmitting applications like television transmitters, the shield is a rigid or flexible metal pipe containing a compressed gas such as nitrogen, and the internal conductor is held centered with periodic plastic spacers. It is a type of unbalanced line, the shield conductor is usually connected to electrical ground. Coaxial cable's advantage is that the enclosing shield conductor isolates the cable from external electromagnetic fields, so it is very immune to interference.