[hpsdr] [Hpsdr] Call for Comments - ALEX

[Ray Anderson]

Chris-

Absolutely!

Like I said:

>You need to make an engineering 
> judgment on what is the lowest frequency magnetic field you need to 
> have good shielding at and then select the shielding material to get 
> what you want.

If system considerations show that external magnetic fields aren't an
issue, then don't worry about it. If there is concern, then certain
factors need to be considered.

In any case, even if mag shielding isn't needed, I think we still need a
box of some sort to put the preselector in rather than just leaving it
hanging in the breeze if only for mechanical considerations.

-Ray  WB6TPU

-----Original Message-----
From: Chris Albertson [mailto:chrisalbertson90278 at yahoo.com] 
Sent: Thursday, July 26, 2007 9:27 AM
To: Ray Anderson; hpsdr at hpsdr.org
Cc: Chris Albertson; Ray J; Ray Anderson
Subject: RE: [hpsdr] [Hpsdr] Call for Comments - ALEX



What is the receivers senitivity to low frequency signals,
the ones that can get through 0.06 inches of aluminum,
say below 100 Khz?  The ADC will be sampling at about 100 MSPS
and then this data will be digitally processed. 

My guess is that no-shielding at all is required
Can anyone think of a radio that is built with magnetic
shielding around its front end band bass filters?

If this is needed then certainly we have to worry a lot about
other more parts of the system like the D to A converter
and even the power supply cables
Or what about that coax cable that only has copper braid?
Looks like some mu-metal conduit might be required.

You don't want to fall into the trap that some audiophiles do.
These guys are nuts and think they can hear the difference
between speakers wire made from silver plated copper vs.
plain copper. 



--- Ray Anderson <ray.anderson at xilinx.com> wrote:

> Chris Albertson wrote:
> 
> >If the magnetic field is non-static or varying with time and has
> >a frequency up in the RF range then it is coupled with an eletric
> >field.  Maxwell proved this a long time ago.  So if you believe
> >the M and E fields are coupled, then what happens is the E field
> >causes a current in the aluminum walls of the enclosure.  We all
> >know that a current will generate a magnetic field.  But we are in
> >luck because the generated magnetic field is exactly opposite of
> >the M field that is always coupled to the E field so inside the
> >box the two cancel. Faraday was the first to notice this effect
> >and we name this a "Faraday Cage" after him.
> 
>  
> 
>  For low frequency H fields (magnetic) the material and thickness are
> important. The material properties (primarily mu and conductivity)
> and
> the material thickness define what is considered a 'low' or a 'high'
> frequency.
> (The higher the mu and the thicker the material the lower the
> effective
> shielding frequency.) You need to make an engineering judgment on
> what
> is the lowest frequency magnetic field you need to have good
> shielding
> at and then select the shielding material to get what you want.
> 
> 
> 
> Excerpted from http://en.wikipedia.org/wiki/Faraday_cage :
> 
> A Faraday cage will not shield its contents from static magnetic
> fields.
> However, rapidly-changing magnetic fields create electric fields in
> accordance with Maxwell's equations. The conductors cancel the
> electric
> fields and therefore the changing magnetic fields as well. The wall
> materials' thickness and skin depth set the frequency at which the
> cage
> suppresses electromagnetic fields. Static or slowly-changing magnetic
> fields penetrate the cage; rapidly-changing ones do not.
> 
> 
> 
> -Ray  WB6TPU
> 
> 
> 
> 


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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