Why a conductive shield doesn't block the magnetic field?

[nabil25]

Hi
While studying the shielded wires, i noticed that the magnetic field of the inner conductor can penetrate the shield conductor (can be calculated in the region 3). However, the boundary condition of the magnetic field at the surface (between dielectric and perfect conductor) of a perfect conductor impose that the tangential component of H (Ht = Js surface current density) and the normal component is Hn = 0. Inside the perfect conductor, we have Ht = 0 and Hn = 0.

So why do we superpose the magnetic field of the inner conductor and the outside conductor when calculating the magnetic field in the region 3 ? Why we still have Hi (inner cond) in the region 3 ?

 

[mfb]

and the normal component is Hn = 0

Where does that come from?

It is true for superconductors - they do provide shielding, but I don't think your question is about those.

 

[Svein]

Leaving theory aside, practical signal cabling theory says: "A coaxial cable shields against electrostatic noise. To shield against electromagnetic noise, use a twisted pair."

See https://en.wikipedia.org/wiki/Category_5_cable for a description of a cable that is designed for immunity against electronic noise.