[Hpsdr] Call for Comments - ALEX

[Ray Anderson]

>> I don't think the shielding of aluminum is "High Performance" for
>> EMI/HF
>> signals, at least not for the magnetic field part of it.

>What did Michael Faraday have to say about this? 

>As I remember a hollow conductor will shield the interior from
>electromagnetic fields.   All it has to do is conduct.  A good
>example of non ferro magnetic shield is the braid on coax cable.
>The way it works is there is current induced in the shield that
>is opposite and exactly cancels the exterior field.  As long as the
>magnetic field is not static the box wil take care of it.
>But we don't care about static feilds they don't couple to the
>electronics inside.

>Sad part is that I used to remember the mathematical derivation
>of this.

>Chris Albertson
   
The aluminum box can effectively shield the electric field, but thin
aluminum won't do much for the near field magnetic field component.

You need a material with a relatively high mu (permeability) like steel
or other ferromagnetic alloy to provide useful magnetic shielding. The
thickness of the metal is also a key factor (thin metal is less
effective than thicker metal).

See chapter 11 in Clayton Pauls 'Introduction to Electromagnetic
Compatibility' for a good discussion of the topic.

-Ray  WB6TPU



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