[hpsdr] [Hpsdr] Call for Comments - ALEX

[Ray J]

Yes I also fail to see how a non magnetic type material (aluminum)would 
shield in/out magnetic fields...

this is why shielded speakers are usually so heavy compared to non 
shielded ones... steel / iron  cans around the speaker magnet...

Ray J
W9RAY



Henry Vredegoor wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi Chris, All,
>
> Thank you for pointing me to the Wiki.
>
> I agree that the shielding of ELECTRIC fields can be done by any good
> conducting material.
>
> But I think shielding of DYNAMIC MAGNETIC fields (like in EMI) can
> practically best be done by using some magnetically conductive materials.
> Since we are using a lot of inductors of powdered iron O-cores, I assume
> that dynamic magnetic fields are predominant in the filter boards?
>
> So, IMHO, I still think steel would be better, although I learned from the
> discussion that it is less of a factor (wrt. frequency) than I thought.
>
> I think one should also differentiate between conducted EMI/shielding and
> induced EMI and shielding in reasoning.
>
> 73's,
>
> Henry.
>
>   
>> -----Original Message-----
>> From: hpsdr-bounces at lists.hpsdr.org 
>> [mailto:hpsdr-bounces at lists.hpsdr.org] On Behalf Of Chris Albertson
>> Sent: woensdag 25 juli 2007 21:04
>> To: hpsdr at hpsdr.org
>> Subject: Re: [hpsdr] [Hpsdr] Call for Comments - ALEX
>>
>>
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>>
>> --- Henry Vredegoor <henry.vredegoor at gmail.com> wrote:
>>
>>     
>>> ***** High Performance Software Defined Radio Discussion List *****
>>>
>>> Hi Chris, Ray, All,
>>>
>>> This is what I meant.
>>> Open up a piece of equipment and if there is any metal shielding
>>> used, it is
>>> most probably tinned steel for this exact reason. 
>>> (Use a magnet to test this.)
>>>       
>> It's easy to see what material is used but harder to
>> see why it was selected.
>>
>> Current prices:  Aluminum = $2,700 per ton, cold rolled
>> electro galvanized steel $700 per ton.  That pretty
>> much explains why they use steel.  Steel is not better
>> then aluminum as a magnetic shield, it's just cheaper.
>>
>> If you did want to shield from static magnetic field you
>> would want something with very high permibility and a bit
>> thicker than sheet metal.
>>
>> Shields don't stop fields the way lead stops xrays or kevlar
>> stops bullets.  Aluminum works because the current in the
>> skin of the metal box itself generates a electromagnetic
>> field inside the box that exactly cancels the one outside.
>> The electronics inside the box "sees" the sum of the two
>> fields, nothing is "blocked"  so any conductive material will
>> work.  Conductive paint is common
>>
>> Here is a good introduction.
>> http://en.wikipedia.org/wiki/Electromagnetic_shielding
>>
>>
>> Chris Albertson
>>   Home:   310-376-1029  chrisalbertson90278 at yahoo.com
>>   Office: 310-336-5189  Christopher.J.Albertson at aero.org
>>   KG6OMK/AG
>>
>>
>>        
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