This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Designing a Faraday cage

I know this isn't a wiring regs question but does anyone know how to undertake the design calculations for a faraday cage to  Min Spec >80dB attenuation 20kHz – 200MHz


This is for a rooms that’s 3 x .3.6 m  


Thanks 


Matt
Parents
  • yes.

    have you looked at the 'light reading' link ? The formulae you need are in there, basically metal thickness is driven by SE, which is directly relatad to skin depth  at the low freq end.  Then it is simply a case of either measuring or looking up the  properties of the permeability and conductivity of the materials you have chosen to make it from. 

    Mesh sizes are an approximation, normally for a quick analytical formula you consider the two polarisations separately, and analyse it as a semi-infinite periodic array of conducting cylinders of regular spacing.  Numerical modelling to include the kinks in the wires at the cross-overs shows this can be up to  about 6dB out near the HF end where it is getting leaky - when the holes are larger than 1/20 of a free space wave roughly. 

    By far the largest source of error wen looking at SE is not knowing the correct  wave impedance to assume at low frequencies - where the free space wavelength is km long, you never have a plane wave, and the ratio of E to H is not 120pi ohms at all, but has far more to do with if the origin is electrostatic - high on E low on H, or a current loop, low on E high on H.

    This is how magnetic coupling works through stainless steel of a few mm at audio frequencies, despite it being an excellent shield to E fiel and to plane waves at that freq..  


    Another good read for practical methods is chapter 8 of Mil Hdbk 419A   old version, but physics unchanged.  also chapter 10 for apertures and penetrating cables Don't expect it all to be obvious in 24 hours, there is the life's work of several teams to absorb if you want to be able to do a full design from scratch - hint, there is a reason most folk do not.

Reply
  • yes.

    have you looked at the 'light reading' link ? The formulae you need are in there, basically metal thickness is driven by SE, which is directly relatad to skin depth  at the low freq end.  Then it is simply a case of either measuring or looking up the  properties of the permeability and conductivity of the materials you have chosen to make it from. 

    Mesh sizes are an approximation, normally for a quick analytical formula you consider the two polarisations separately, and analyse it as a semi-infinite periodic array of conducting cylinders of regular spacing.  Numerical modelling to include the kinks in the wires at the cross-overs shows this can be up to  about 6dB out near the HF end where it is getting leaky - when the holes are larger than 1/20 of a free space wave roughly. 

    By far the largest source of error wen looking at SE is not knowing the correct  wave impedance to assume at low frequencies - where the free space wavelength is km long, you never have a plane wave, and the ratio of E to H is not 120pi ohms at all, but has far more to do with if the origin is electrostatic - high on E low on H, or a current loop, low on E high on H.

    This is how magnetic coupling works through stainless steel of a few mm at audio frequencies, despite it being an excellent shield to E fiel and to plane waves at that freq..  


    Another good read for practical methods is chapter 8 of Mil Hdbk 419A   old version, but physics unchanged.  also chapter 10 for apertures and penetrating cables Don't expect it all to be obvious in 24 hours, there is the life's work of several teams to absorb if you want to be able to do a full design from scratch - hint, there is a reason most folk do not.

Children
No Data