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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
  • Former Community Member
    0 Former Community Member
    Do you need to design it from scratch - based on your quoted dimensions and degree of attenuation at the stated frequencies, you're after a pretty standard steel lined enclosure (or a 3 - 5mm steel box) with a knife edge door and a few honeycomb wave guide entries. Possibly jacked up on a frame to allow inspection underneath it, and robustly earthed to a reliable ground plane. Depending on what services you need you may have to use a few tempest filters on some of the incoming circuits, and consider the cable type (shielded with foil plus braiding would be usual) - generally keep as much out of the box as you can.


    Basically, if you can define the E field and H field, then for any frequency you can quickly work out the shielding thickness required for a given material.


    You could effectively buy (or specify) one "off the shelf" at 80dB attenuation. You may need a mix of ferrous and non ferrous linings if the 80dB is absolute at all specified frequencies, aluminium/steel composites are particularly effective at absorption and reflection techniques. There are a few companies listed via CPNI (but you may need a security consultant or similar to access the data) and a whole lot more knocking about for less secure applications


    You can design from scratch but eventually, you'll run into performance certification problems if you have to satisfy the requirements of a Class consultant or similar acting for the ultimate client - for example, you usually won't have any drama with an enclosure certified to, say, MIL STD 285/IEEE-299


    If Mapj1 rocks up, this will be right up his street if you do want some outlines for designing one. I'm a lazy sod, I'd just buy one ?


    Regards


    OMS


  • Beaten to it. bah!


    Actually 80dB  is not an especially  onerous a spec,  a biscuit tin soldered shut round the seam will do that, until you start adding penetrations for services - do you need water, vacuum or gas  for example, or mains or lights?

    Of course they do not make biscuits that big, as they would be hard to eat, and dashed tricky to cook as well probably, but you get the idea.


    I'd suggest you chat to one of the suppliers of off the shelf solutions such as ETS Lindegren or MDL or visit someone with a chamber and just look at it.

    If you have an excess of labour over budget (university dept for example) I know a few outfits have successfully built their own.

    At that spec you could have up to about a foot square or so of "double glazed"  (i.e. 2 layers of mesh about an inch apart) mesh window to peer through if you like mesh


    Do you need it to be absorptive of RF or is internal reverberation acceptable ?


    The former requires absorbent material to be placed and tuned for the bands of interest, and for wide bandwidths you have to accept a very small quiet spot where it is properly anechoic .


    A reverberation chamber however is just a metal box. Our 4m cube one is double skinned 1,2mm thick tinplate laminated on either side of 25mm plywood in sections bolted with gib strips and mesh inserts in the joins to make the walls floor and roof , as this allows them to contain a reasonable blast if something fails, absorbing energy by splintering the wood, as well as stopping photons long and short with the metal.

    Door seals will indeed be meander channel and finger stock finger stock  door seals   . The force to close is considerable, if more than about 1m by 2m they will probably need gas assist to shut properly - fingers out !.

    What is your peak power handling - will it need air extraction or cooling water ? (honey comb is the key for air, and consider waterpipes as dielectric loaded waveguides to set your maximum diameter If you need a lot  of water and so need large bore pipes, consider an all metal plumbing circuit and  heat exchanger so that the water going outside is fully shielded from the live water inside) .

    Note that lights will need to be behind mesh, especially if you are doing EMP work - or the lights tend to blow out during testing. That is not an issue at low powers - but if you need a quiet room for listening tests then consider filament lamps powered by DC as less likely to radiate and interfere with measurements.


    You may need ramps to get heavy things over the door sills and a pulley in the roof centre for setting up tall stuff.

    If there are cables in and out for instrumentation and command  they should leave through a single access panel (or if there must be two they should be in the same side please ) that can be removed to have filtered bulkhead connectors or a small box fitted to either side of it for each wire to be filtered and trapped.  Do not let anyone drill right through the walls, or bring an untrapped wire in, so make it easy to do it right by making good provision.

    An emergency call switch can be string pull through a meander line of metal tube in good electrical contact with the wall as it penetrates, wiring outside, string on inside.  No telephone unless in a shielded box that stays shut during tests !.


    Once many experiments have had a go, and the access panel looks like a Swiss cheese  it can be replaced with a new piece of plate  for modest cost and the chamber is as good as new..



    just some out of order ideas - are you able to say what is it for ?

    light reading
  • Former Community Member
    0 Former Community Member
    LoL - right on time,  Mike ?


    Lets hope it's not for the military attaché in Moscow or Beijing, other than that, a half sized ISO container with a new door should suffice  - very metropolitan ?


    Regards


    OMS


  • ISO containers are a bit smaller than that  - more like 2.5m by 2.5 by 6, rather less inside, and at least one I am  'aware of'  ended up with big  holes cut to put things in and then being welded up and repainted  afterwards, due to a "£$"%% up, (or a  dimensional miscalculation in the language of the final report) with the relationship of  size of the equipment to the door aperture.

    They do need a bit of rework to make them into decent screened boxes, but are a jolly solid starting point in terms of rigidity, and ease of planting them on-site..
  • Hi there, 

    Yes i need it from scratch, ideally how you work out the sizing of the mesh, the thickness and the type of construction, copper, steel etc.


    Has anyone any ideas? 



    Matt 

  • 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.

  • Former Community Member
    0 Former Community Member

    mapj1:

    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.

     



    For the OP, as I suggested, engage someone who does this for a living  - that won't stop you attempting to work out yourself why they have selected what they did as a self directed learning exercise - but if this enclosure ultimately has a genuine purpose or function, then how are you going to warranty your design to get client acceptance.


    Regards


    OMS
     

  • Thanks guys


    Ive reached out to MDL so lets see what they have to say, hopefully they can give me an idea of cost too.  





  • Depending on details of fit out in terms of no of doors and panels, if it needs to be made in small sections because of the location, and who is installing (you or them) and what on-site tests you need,  as a 'ballpark'  to avoid sudden shocks I'd suggest you set aside from £45k to £90K for a 4m cube. When they get back to you, please let me know if I'm now miles out of touch - it has been nearly a decade.
  • Former Community Member
    0 Former Community Member
    In the right ball park, Mike


    I put a 100dbA unit in about the same size, for a Facility that occasionally needs to communicate with the wider world with a degree of discretion - bit of an awkward location so provided in several bits - towards the upper bound of your estimate.


    Regards


    OMS