coaxial cable

Unscrew the round connector behind a TV and you will meet coaxial cable: a single copper wire running down the dead centre, wrapped in a layer of insulation, then sheathed in a tube of braided metal mesh, all inside a plastic jacket. 'Coaxial' means the inner wire and the outer mesh share the same central axis — one nested inside the other, like a pencil inside a drinking straw.

That nested design is what makes coax good. The signal travels on the inner conductor; the outer metal mesh serves double duty as the return path and as a shield that surrounds the signal completely, walling off outside interference and keeping the signal's own energy from leaking out. Because the geometry is so controlled, coax has much greater usable bandwidth than twisted pair and resists noise far better, letting it carry high frequencies over longer distances — which is exactly why it became the medium for cable television and cable internet, where dozens of channels share one cable by frequency-division multiplexing.

Coax sits between twisted pair and fiber: more bandwidth and noise immunity than copper pairs, but heavier, stiffer, pricier, and ultimately far short of fiber's reach and capacity. It dominated early Ethernet (the original 10BASE5 'thick' and 10BASE2 'thin' coax buses) and still rules the last hop of cable broadband into homes, but for backbones and long runs it has yielded to optical fiber. Its specified impedance matters: TV/cable coax is 75 ohms, and old Ethernet coax was 50 ohms — mixing them causes reflections and errors.