[hpsdr] [Hpsdr] Call for Comments - ALEX

[Ray Anderson]

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




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