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 name "common mode" means that the current on the shield and the center conductor is traveling in a common direction. This condition is a violation of the assumptions required to make a transmission line work.

For the transmission line to work correctly, the current must be in opposite directions on the shield and center conductor; in this case, the current will be on the inside of the shield and the outside of the center conductor, the induced magnetic fields will oppose and cancel, and there will be no leakage.

If there is common mode current, then these conditions are violated, and there will be RF leakage both into and out of the transmission line.

The world generally not being perfect, it is possible for both conditions to exist simultaneously. In other words, you can have both common mode current and opposing current on the shield at the same time. When this occurs, then presumably the common mode current will be on the outside of the shield and the opposing current on the inside of the shield and the portion of the current that is common mode will cause leakage. However, it would also be possible (but unlikely--so maybe only a small fraction of the current) for the common mode current to travel in other parts of the shield and cause eddy currents, which would heat the shield rather than leak. Poor quality coax might have a thin shield, which be less than the skin depth, allowing the two currents to mix and allowing eddy currents to be worse.

Typically, common mode current in transmission line is caused by the shield of the coax coupling with the antenna, so that matched opposing currents flow in the antenna and the outside of the shield in addition to the matching currents on the inside of the shield and the outside of the center conductor. This coupling essentially makes the shield part of the antenna.