A coaxial cable has an inner conductor surrounded by a concentric outer conductor, with dielectric between them. In the ideal homogeneous lossless Transverse electromagnetic mode, electric field is radial and magnetic field circles the axis; the return current flows on the outer conductor. For inner radius and inner shield radius , its characteristic impedance isFor air dielectric this is approximately ohms. A cable's characteristic impedance describes travelling waves, not its DC wire resistance. Finite conductivity, imperfect shielding and higher modes limit the ideal description.
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