Common mode currents and double shielded coax

[A.Z.]

Folks, I'm banging my head about this, but can't reach an answer; I know about the three conductors in a coax and the CMC flowing on the outside of the coax braid, but ... what if we have a double shielded coax, where we have an external braid, insulator, an internal braid, center insulator and then center conductor, what happens to CMC then ? Could anyone here help this old brain sorting out this question ?

[joeqsmith]

Same effect.  Common mode will want to be towards the outside. 

[A.Z.]

Same effect.  Common mode will want to be towards the outside.

yes, but the outside of... which braid ? With a dual shield coaxial we'll have 5 "conductors", the inner and outer surfaces of the outer braid, the inner and outer surface of the inner braid and the outer surface of the center conductor

[joeqsmith]

Away from the center conductor.

[A.Z.]

Away from the center conductor.

As you wrote it, your reply makes little sense, but thanks anyway, found the answer myself; the CMC will flow on the outer shield of the external braid due to the skin effect which I didn't consider

[T3sl4co1l]

If they're all shorted together at either end, then at worst, a standing wave mode would be present between the two shields.  Say with triax cable.

Screened and shielded stuff, you can safely assume the layers are shorted together throughout the build, all behaving as one conductor.  And yep, that keeps the currents separate, CM on the outside, DM on the inside, separated by skin effect.

Indeed, a tight braid, and foil layers, and more of both in the case of some types, improve the conductivity of the shield and thus make the skin depth shallower, while also increasing the shield thickness, increasing isolation between modes.

Tim

[A.Z.]

If they're all shorted together at either end, then at worst, a standing wave mode would be present between the two shields.  Say with triax cable.

Screened and shielded stuff, you can safely assume the layers are shorted together throughout the build, all behaving as one conductor.  And yep, that keeps the currents separate, CM on the outside, DM on the inside, separated by skin effect.

Indeed, a tight braid, and foil layers, and more of both in the case of some types, improve the conductivity of the shield and thus make the skin depth shallower, while also increasing the shield thickness, increasing isolation between modes.

Tim

Hi Tim

Thanks, but now you bugged my brain with another question; let's say we have a run of double shield coax, and let's say that we ground one shield at one end ONLY and the other shield at the opposite end ONLY

now, the inductance, resistance and capacitance will surely play a role here...

ok, it's just a mind game at this point, yet it's interesting, I believe

[T3sl4co1l]

With triax?  Or the stuff posted above?

Tim

[coppercone2]

I never saw coax that has a shield split, that would be a capacitor for no reason. In fact, this would define it as a triaxial cable. Triaxial cable I don't see specified for more then a few hundred MHz (cameras). with exceptions like this
http://www.clarkwire.com/cableTV7511DB.htm

The foil is pressed into braid by constricting shrinking insulation (i think its thermal based, like heat shrink or just a extrude) to offer a pretty good low resistance.

A triaxial cable will form a capacitor couple if connected that way, the equation is the coaxial equation with the outer shield and inner shield diameters.

If you ever saw coaxial cable used as a capacitor, the same thing will happen between the insulated shields. Because of faradays law, the inside conductor is fully screened so you can imagine the cable as the inner shield being solid and the outer shield being a pipe. Then you have leakage because of the braid which is complex analysis.

You can get a LCR meter and measure the capacitance between everything in the cable if you want.