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Registered Member #4252
Joined: Fri Dec 09 2011, 10:43AM
Location:
Posts: 18
Hello guys
I am designing a project which includes high voltage discharges do deform metal pieces. I thought about using the box around my tool-coil as a Faraday cage. I am reading about the phenomenon involved and the cage works with and external field, eliminating the internal field. In my case, the field would be generated inside the cage and the objective was to isolate outside equipment and workers from danger. Do you think this configuration might work?
As long as the meshing space is smaller than 1/10th the wavelength and it is connected to a good RF ground it should block all RF emissions. That means what's in - stays in, and what is out, doesn't get in.
Registered Member #4252
Joined: Fri Dec 09 2011, 10:43AM
Location:
Posts: 18
Sigurthr wrote ...
As long as the meshing space is smaller than 1/10th the wavelength and it is connected to a good RF ground it should block all RF emissions. That means what's in - stays in, and what is out, doesn't get in.
Hello Sigurthr.
Thank you for your help. Could you explain a bit more that 1/10th rule?
Sure. Eh, it's not quite a rule, and it has been some time since I designed a faraday cage (and that was for radio signal containment, not HV, but what works for one works for another). Generally when working with RF you want to avoid any lengths longer than 1/10th of a wavelength if you don't want that part/wire/conductor to radiate energy (or rather, radiate it efficiently, as EVERYTHING conductive radiates some amount).
I can't remember the exact fraction of a wavelength a mesh has to be to to effectively block a signal, but I do know from experience that 1/10 lambda does work. It probably would work at 1/5 lambda (lambda means wavelength, and is less to type =P ), but I'm not going to say so because I don't know for sure. I do know for sure 1/10 works.
Basically the inside diameter of corner of the mesh grid to the opposing corner has to be less than 1/10 lambda. So for example, to contain a 2meter signal (146MHz) the largest distance between the metal of the mesh cannot be more than 20cm.
lambda in meters = 300 / frequency in MHz
So to protect against anything lower than 1GHz the mesh has to be 3cm or smaller. You follow?
Remember, it only works if the cage is tied to a GOOD rf ground. If the ground conductor is high impedance at the target frequency it won't work. Also, the ground lead can radiate if it is longer than 1/10 wavelength (sometimes unavoidable, so don't stress over it).
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
This 1/10th rule doesn't make much sense for tesla coil frequencies. 50khz has a wavelength of 6km, and I really doubt metal rods 600 meters apart would block much of anything out of a tesla coil.
I think in this case you need to just use common sense as to mesh sizing.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Any discharges of an electric spark will produce high frequency signals in metalwork which may have formed resonant circuits that are determined by shape, and depending how much energy you are using, the Faraday cage may compound the problem. You might consider an anechoic chamber for E/M protecting your workers.
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Faraday cage
