FreeLists / si-list / [SI-LIST] Permeability Affects?

[Chris Padilla <cpad@xxxxxxxxx>]

Being an EMI engineer for a few years, I tend to think in terms of 
permittivity and electric fields but certainly permeability and magnetic 
fields are equally as important and while I do consider the latter quite a 
bit, I've never thought a whole lot about permeability--at least not on 
purpose or directly!

I received a question from a connector's vendor (like Tyco/Amp, Molex, 
etc.) wondering if the permeability of his shield material was important or 
not in helping/hindering emissions.

As an example, if one take a piece of iron (muR ~ 7000) and shields a 
magnetic compass in a decent magnetic field, the compass will be unaffected 
by the field.  Iron, being a ferromagnetic, becomes magnetized and the 
field flows within the material but not through it.  This means we have 
current flowing in a particular direction within or on the surface of the iron.

If a connector vendor's shield (as an example, consider a multi-port RJ45 
connector used in ethernet applications) has a high permeability, and I 
have current flowing on that shield, I have a strong possibility of 
radiation occurring.

Perhaps the key here is providing a path for the current to go elsewhere 
rather than on the shield, which is why the shield gets grounded to my 
chassis and to the planes within a PCB.

I just wanted to toss this thread out there....


Thanks----->Chris

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