Question: People [often say](https://ham.stackexchange.com/search?q=outside%20of%20the%20shield) common-mode current flows on the outside of the shield, while the current on the inside of the shield is always opposed by an equal but opposite current on the center conductor. This is because skin effect separates the inside and outside of the shield.

But if skin effect is isolating the inside and the outside of the shield, why would the inside-shield and center-conductor currents *always* be equal but opposite? If skin effect makes the inside and outside of the shield distinct, why can't there be a common-mode current on the inside?

Answer: 
First, does it matter where the common-mode currents flow? Coax works as a transmission line because the (ideally) equal but opposite currents on the center and shield create opposite fields which exactly cancel for points outside the cable. Thus, it does not radiate. And by reciprocity, external interference is rejected.

The same concept is in play with [twin-lead transmission lines](https://ham.stackexchange.com/q/3508/218): the equal and opposite current cancel. No skin effect required.

*Skin effect does not make the inside and the outside of the shield function as distinct conductors*. Think about the contradictions this would make: how does current "know" if it should flow on the inside or the outside? How would open transmission line stubs work if the shield current on the inside "wrapped around" and traveled back on the outside upon reaching the end of the coax? [How could twin-lead choke baluns work?](https://ham.stackexchange.com/q/6983/218)

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There's a phenomenon similar to skin effect called [proximity effect](https://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)). It's relevant to transformers and twin-lead, or any application where two or more conductors in proximity are carrying RF current.

When two conductors are carrying currents in opposite directions (differential-mode), the current density is highest in the regions closest to the other conductor. When the current is in the same direction (common-mode), current density is highest in the regions farthest apart.

[![enter image description here](https://i.stack.imgur.com/rJl65.png)](https://i.stack.imgur.com/rJl65.png)

Both skin and proximity effects arise due to [eddy currents](https://en.wikipedia.org/wiki/Eddy_current). Skin effect is a special case when there's just one conductor, with the current density trying to get as far away as possible from all the other parts of the conductor trying to carry current in the same direction.

So in the case of coax, for a common-mode current the current density will be highest on the outside of the shield since that's the farthest point away from all the other points in the coax.

For a differential-mode current the current density is highest on the inner surface of the shield and the outer surface of the center since that's the closest points between the two.

[![enter image description here](https://i.stack.imgur.com/NYj6b.png)](https://i.stack.imgur.com/NYj6b.png)

This isn't skin effect creating a magic barrier between the inside and outside of the shield. It's eddy currents reinforcing some currents while cancelling others. It's the same underlying physics as skin effect, but there's no magic barrier. The inside and outside of the shield are not "distinct conductors".

And that the current density is highest on the outside of the shield isn't relevant for most purposes. It's cancellation of fields, not magic skin effect barriers that are responsible for making our coax not radiate.
The statement "common-mode current flows on the outside of the shield" is true for a perfect shield. A low cost 60 ohm TV cable might have some common-mode current on the inside. Look at it the other way, Inject a common-mode current on the coax by use of a toroid transformer and measure the voltage between screen and inner conductor at the cable end. A perfect cable (Flexwell with 0.5 mm wall thickness copper tube for the screen) would not show any signal at all, but a low cost TV coax might show something like 60 dB below the coupling to an un-screened wire.

* leif -