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let's consider a coaxial cable with only two conductors (the core and the external metallic shield), and let's consider the case in which it propagates TEM mode at a certain frequency:

enter image description here enter image description here

My question is: how does the shield operate in order to get 0 (or near 0) electromagnetic field outside the cable)? I always have been told that its shielding effect is due to the shielding property of very good conductors. I may understand this explanation, but I have a doubt: along the metallic shield there is a current which is equal and opposite to that flowing along the core. So, it should generate a circular magnetic field also outside the cable.

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My question is: how does the shield operate in order to get 0 (or near 0) electromagnetic field outside the cable)?

The external magnetic field generated by the returning current in the shield is exactly cancelled by the opposite external magnetic field due to forward current in the inner (or core).

There will be a magnetic field between shield and inner (core) though.


I've previously done this task using QuickField to show the magnetic fields in two scenarios. The coloured contours show flux density B: -

enter image description here

The models are a little crude because this is the student edition of QuickField and it doesn't give you many free finite element nodes to play with hence, the model is a tad clunky, restricted in boundaries but, delivers the goods for this question.

The first scenario shows the magnetic field surrounding the shield when 1000 amps exclusively flows through it at 100 kHz. Notice that there is no flux produced inside a current tube (as per classical physics) but there is still induction to the inner core (96.3 volts)

The bottom scenario is the same applied current but exclusively flowing in the inner. Notice that is produces flux all around the inner core and, if you look in the boundary corners at the colour change, it is the same in both scenarios thus implying flux density is the same.

The decisive number - the voltage across the shield (per metre) in scenario 1 is 95.9 volts and, in scenario 2, the voltage across the shield (due to induction from the inner) iis 96.4 volts (per metre). In a perfect world, with a perfect model, with infinite boundaries, those two voltages would be the same thus indicating that: -

$$\color{red}{\boxed{\text{The external fluxes are the same but opposite for opposing currents}}}$$

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  • \$\begingroup\$ So is it not due to the shielding property of ideal conductors? Is it just a cancellation of fields \$\endgroup\$ – Kinka-Byo Jun 22 '20 at 13:30
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    \$\begingroup\$ @Kinka-Byo I've added some more data to my answer - I think you'll find it informative and basically tells you it's magnetic field cancellation. Been down this path and argued with physicists and chemists who grumpily (and eventually) said OK with no thanks LOL! \$\endgroup\$ – Andy aka Jun 22 '20 at 13:46

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