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I was wondering how the current flows in a coaxial cable. Say for instance that the inner conductor transfers the signal, so there flows a current. How will in the outer conductor(shield), flow a current with a 180degrees phase shift compared to the inner conductor.

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  • $\begingroup$ What precisely is "the signal"? $\endgroup$ – Phil Frost - W8II Jun 15 '18 at 12:36
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The RF current in a coaxial cable flows from the signal source (e.g. a transmitter), through the center conductor to the load and then back through the outer conductor of the coax, to the signal source. This is the conventional current flow description since the outer conductor is generally grounded at the signal source.

The 180° phase shift of the current on the outer conductor exists simply because the current is flowing in the opposite direction compared to the center conductor. For a coaxial cable to function as a transmission line and not an antenna, not only must the shield current phase be 180° from the center conductor current but the current in both must be of equal amplitude. If either of these conditions are not met, the coax will radiate.

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