This is a common theme from several of my talks over the years about Ferrite Chokes and Baluns. By expanding the topic into a blog format, I hope to be able to explain it in more detail, and add some FAQs as time goes on.

This well-known image originated with Walt Maxwell W2DU, the author of many articles about transmission lines. It brings out several important points.

• The inside of a coaxial cable truly is “private” – completely screened from the outside world. This is one of the rare statements that can be handed to absolute beginners as a simple fact, and still remains valid all the way through to graduate-level EM physics.

• The private interior of coax is due to the skin effect which means that, at RF, current flows only on the surfaces of conductors.
  
  There are many confusing illustrations in textbooks that make some people imagine the skin effect only applies to electrical conductors of some particular shape and size, or in some particular type of circuit. That isn’t true! The best and most generalized derivation of the skin effect I have been able to find is reproduced here. All the high-level EM physics and maths leads to a simple and very powerful conclusion:
  Whenever and wherever an RF current is flowing (regardless of the circuit configuration, regardless of the reason) the skin effect will always be present.
  RF current will always be forced to flow very close to the outer surface of the conductor. There is no net current flow into or through the thickness of the material. 
  
• Because we know the skin effect will always be present, we can use that as a starting point when tracing RF current pathways on complex shapes such as coaxial cables and shielded loops. Suddenly, the whole topic of shielding (and what you need to do to make it work correctly) will begin to make sense!
  
  There is one condition, though: you have to hold onto that idea consistently. Many of the myths in RF engineering arise because people aren’t being logically consistent. They imagine they can switch these basic ideas on-and-off to suit their own preconceived notions… and physics just ain’t like that.  

• In coaxial cables, the skin effect in coaxial cables means that the outside surface of the shield is a completely separate conductor from the inside surface.
  
  The outside surface of the shield is part of the outside world. This is where the so-called “common-mode” currents flow.

Those are a lot of observations to keep in mind, but they all lead to one simple conclusion.

Summary

• Because the currents I4 and I5 in the antenna are never quite equal, they can be mathematically resolved into a differential-mode component and a separate common-mode component. 
  
  
• However, a coaxial feedline will physically do the same thing! At RF, where the skin effect creates a third conductor on the outside of the shield, the pure differential-mode component of the incoming two-wire connection will automatically be routed into the private, shielded interior of the coax to become I1 and I2. Meanwhile the common-mode component is routed separately onto the third conductor, the outside of the shield, to become I3.

And that’s the private life of coax.