[hpsdr] [Hpsdr] Call for Comments - ALEX

[Lawrence D. Lopez]

A perfect conductor does not allow entry of a magnetic field.
http://en.wikipedia.org/wiki/Lenz's_law

But it isn't that simple.

Airframes take damage when they are struck by lighning
by induced fields.

I don't know the answer.

I do know mu metal is used to shield CRTs.
As opposed to aluminum.

Ray J wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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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