Re: faraday cage questions (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Tue, 26 Dec 2006 02:50:08 -0800 (PST)
From: davep@xxxxxxxx
To: hvlist@xxxxxxxxxx
Subject: Re: faraday cage questions (fwd)

[best of the holidays to all]

> functioning after ive had my fun!  Since i would still love to work on
> my HV projects at school, i have considered building a faraday cage. The
> design has to be easily collapsable for when im done experimenting and
> need the space again.
      Having been involved in one 'noise abatment' cage and workes
      in a others and on emi in general...

      Collapsible will not be eays, as will, i hope become apparent/
      Cages have, what may be looked at, as two functions:
         complete strike rail: the bolts can't hit anything
         outside.
         Noise abatement: keeping EMI in (or out (or both))

> Mesh size:  Is it safe to assume that any frequency with a wavelength
> greater than the mesh opening will be completely attenuated?
      The Rule of thumb was longest opening shorter than
      1/10 wavelength, at shortest wavelenght.  The details can
      be dug out by looking at the math for 'slot antennas' and
      the next comment.

      This One Is The Hidden Gotcha:
        The governing dimension is the SINGLE Longest
        dimension. Thus, a door, all metallic, with
        unbonded edges is an opening 6' 'open'.  Bonding
        between edges must observe the '1/10 wavelength rule.'

      That Said: wavelengths of coiling, so 200KHz & down,
      are easier to deal with than 1 GHz & down.

>  Or is
> there some function that describes the attenuation of radiated energy
> through such a mesh size?  I was planning on either using 1/2" or 1/4"
> square heavy duty mesh, but i could resort back to something like
> aluminum window screen, which is something like .08" squares i think. I
> need to avoid interference with our WiFi connection (~2.4GHz).
     cf as above.
     It is my understanding that common sparky sources do not
     get up to GHz range as theya re two slow (reionization time),
     but the devices may be annoyed by slower sources (100s MHz)
     and appear to not work.  Sorting out what ACTUALLY is going on
     can be slow, painful and not practial, socially, in a group
     environment.

> Power feed-through:  What would be the best approach?  I was thinking of
> just using normal line filters, but locating them "mid-way" into the
> mesh and connecting the case to the screen.  But, im not sure how
> effective line filters really are.  Also, what if i wanted to simply
> feed HV into the cage (from a HVDC power source), i suppose i would need
> to design some sort of feed-through filter for that as well
> (ideally i'd avoid this situation).
     All correct.
     Any line going in/out Must be filteres 'in' the cage wall.
     ('doghouse' design is one slang term).
     Any line can carry 'strays' of any frequency.
     Any filter must filter at the relavant frequencies (some,
     hit with broadband noise have leak freqs.
     Isolation Transfromers comein 'filtered/screened' and
     not.  The 'not' kind can couple strays thru interwinding
     capacitance.

     This is not all intended to sound scary, rather, to illustrate
     somethings that may need to be pondered, or troubleshot.

> I plan on running small tesla coils and possibly my marx generators
> inside this cage to give you an idea of what im dealing with.  I'd
> assume that i can just connect the cage to mains ground simply for
> safety reasons, but no RF current will be present on this connection as
> the RF circuit is completely enclosed within the cage.
     Numerical value for 'small'?
      8)>>
     Otherwise, concur, again.
     Minor Point: grounding the cage is irrelavant, if the cage
     IS A CAGE: as you note 'nasties' stay inside.

> Any tips or comments are appreciated.  Im also looking for a cheap
> source for the screening, if anyone has a suggestion.
      Good luck.
       dwp