Coaxial cables are composed of two different identical conductors separated by insulating materials. The inner conductor is copper clad aluminum or copper core wire, and the outer conductor is aluminum pipe or metal braided net sheath. If the outer conductor is grounded, it will act as the shield layer of the central conductor, which has the function of resisting external interference. In coaxial cable transmission theory, it can guarantee the radio frequency transmission within the cable. Due to the close distance between the inner and outer conductors, there will be some cables between them. In fact, inductance and capacitance are evenly distributed along the transmission line. So the characteristic impedance of the transmission line depends on the inductance and capacitance of the line, and they depend on the size of the line.