EMMFLEX coaxial cable (or "coax") is the most common cable used for transmitting video signals. The name "coaxial" refers to the common axis of the two conductors. Construction : A coaxial cable has a solid copper or copper - clad - steel centre conductor surrounded by a nonconductive dielectric, insulating material. The dielectric is surrounded by foil shield/s and/or copper braid/s which form the outer conductor and also shield against electromagnetic interference (EMI). The outer conductor / shield is encased in a PVC jacket. Most coaxial cables for video applications have a nominal impedance of 75 ohms. Their differing electrical and physical characteristics make it important to select the correct type of cable to suit the applications.