Three conductor model for coax

I recently learned how coax can be represented as three conductors: the center, the inside braid, and the outside braid. As we know it's often used to explain common mode current.

It got me wondering about that fat commercial hard line coax where the center conductor is actually a tube. Does that end up acting like 4 conductors? Or does everything stay on the outside surface of the inner conductor.

I am asking only out of pure curiosity.

 

The differential-mode signal is carried between the outside surface of the center-conductor and the inside surface of the shield. The instantaneous current "up" the outside of the center-conductor almost perfectly balances the current "down" the inside of the shield, so very little field "leaks".

It matters not if the center-conductor is solid or hollow. The only RF current that flows on it carried by skin-effect. If the center conductor is a hollow tube, there is no current inside the tube... Cheap coax has a steel center conductor with a bit of copper plated on it...

 

Hey in some cases I pump DC current through the same coax as RF.

Four conductors:

-The entire center and outer conductor areas for DC
-The outer surface of the center conductor for RF (differential mode)
-The inner surface of the outer conductor for RF (differential mode)
-The outer surface of the outer conductor for RF (common mode)

Maybe even five conductors if I count "two" for the DC part.

 

Why doesn't the skin effect cause current to flow on the inside surface of a hollow center conductor the way current flows on the inside surface of the hollow outer conductor?

 

For the same reason that the current that flows on the inside of the shield has does not flow on the outside...

 

I don't know why the same current would not flow on both the inside and outside of a hollow center conductor.

Common mode current on the outside of the outer conductor has, at least in part, the same source as the differential current flowing on the inside of the outer conductor.

 

The interior of an empty tube is only a single conductor. Conventional RF transmission lines require two or more conductors, so an empty tube cannot function in that mode.

The two "non-conventional" exceptions actually reinforce that point:

1. If the frequency is high enough, the hollow tube could be used as a waveguide. But at all lower frequencies, an empty tube becomes a "waveguide beyond cutoff", meaning that RF fields and surface currents cannot penetrate inside.

2. A single wire can support a traveling surface wave (G-line), but not very easily, and only on an outside surface. On an inside surface, G-line won't work at all.

However, coax with a hollow center conductor can very easily be used for simultaneous transmission of RF, DC and air. Design of suitable multi-mode connectors is left as an exercise for the reader.

73 from Ian GM3SEK

 

but it does sometimes. and we call it common mode current.

 

The part that flows on the inside of the shield is caused by the differential mode carrying power from source to load (and if load is not matched to Zo, then the part of the differential mode due to reflection at the load). If there is common mode current on the outside of the shield, it is due to an entirely different reason.

 

Really? The RF current that flows on both the inside and outside of a coax outer conductor has the same energy sources, from the transmitter during transmission and from the environmental RF during reception. Both sources are connected to both the inside and outside of the outer conductor. Because of skin effect the inside and outside of the outer conductor are, at RF, two separate conductors.

The inner conductor, whether solid or hollow (if it is a copper clad steel conductor it functions as a hollow conductor) carries only differential current under normal circumstances. It is shielded from external RF sources and prevented from radiating by the outer conductor(s).

The differential aspect of currents in a transmission line has essentially to do with the proportion of currents which are not being radiated. The differential currents flowing on the center conductor and the inside of the outer conductor do not radiate. The common mode current flowing on the outside of the outer conductor is radiated or induced by RF energy in the environment.