iii. Co-axial Cable

In communications systems, cables commonly consist of numerous pairs of paper-insulated wire, encased in a lead sheath; the individual pairs of wire are intertwined to minimize induced interference with other circuits in the same cable. To avoid electrical interference from external circuits, cables used in radio broadcasting are often shielded with a winding of metal braid, which is grounded. The development of the coaxial cable was an important advance in the communications field. This type of cable consists of several copper tubes; each tube contains a wire conductor that extends along its center. The entire cable is sheathed in lead and is generally filled with nitrogen under pressure to prevent corrosion. It has a single central conductor made of solid wire and is surrounded by a jacket made of Polyvinyl chloride (PVC). Its more frequency spectrum offers stronger transmission. It has multiplexing and broadband facilities. Because the coaxial cable has a broad frequency range, it is valuable in the transmission of carrier-current telephony.