What material surrounds the center conductor in a co-axial cable?

In a coaxial cable, the material that surrounds the center conductor is the dielectric. The dielectric serves as an insulator that separates the inner conductor from the outer conductive shield. This insulating layer is crucial because it ensures that the electrical signals travel along the inner conductor without interference from the outer shield.

The dielectric material must have good insulating properties to maintain the signal integrity and prevent signal loss, while also allowing for the controlled velocity of the signals propagating through the cable. The choice of dielectric can influence the cable's performance characteristics, such as its capacitance and attenuation.

While plastic insulators and various types of foam insulation can be used in some coaxial cable designs, they mostly refer to specific forms of dielectric materials, which can sometimes include solid or foam variations depending on the application requirements. Metal sheaths, on the other hand, are typically associated with the outer layer of coaxial cables and serve as a shield to protect against external interference rather than insulating the inner conductor.