On 8/14/11 7:46 AM, Brian Hall wrote:
A small Faraday cage around the coil might contain the RF so as not to interfere with the radio. From watching the video on the link you provided the discharges aren't that long, so a box type cage with wood for the edges and metal screen door mesh (which you can probably get a your local hardware store) for the sides and top might work. See some of the cages emailed recently to the list, pictures people sent of what they stand inside of while big coils throw sparks at the cage while taking pictures with RF sensitive digital cameras. When I was on a tour of the MIT ion thruster lab they had a similar rig with fine copper mesh around a new project, so as not to interfere with other electronics in the room. Just a guess here, someone else should probably weigh in too before you spend that much on the rig.
Making a cage to stop sparks for safety is a very different matter from keeping RF in/out.
For the former, pretty much anything works and you can have big gaps, power/control wires can go through the gaps, etc.
For the latter, it's much tougher. You need to know what radio might you be interfering with (VHF wireless mic? AM broadcast band? etc.)
A cage will do a fine job stopping electric field radiation, but not very good at stopping magnetic fields. Magnetic fields might be of concern for transients (that fast changing magnetic field from a spark induces a voltage in a loop in the victim equipment), but, as a saving grace, they drop off very fast with distance.
Give the current somewhere to go, other than your shielding. That means, for instance, having an grounded strike point inside your cage, and not letting the sparks hit the cage.
Basic rules on shielding:Perimeter of the hole (or gap) is what's important. You want to be very much less than a half wavelength at the frequency of interest. (note well, the required performance for a mesh reflector is pretty bad from a shielding standpoint. A reflector that reflects 99% of the power is considered quite good, but a shield that only attenuates by 20dB (same performance) is considered quite bad)
Anything that penetrates the "shield wall" is bad. that means wires carrying power or control signals.
Woven wire is bad (because the wires are first inside, then outside, so they penetrate the shield wall). Welded mesh is better. Perforated plate is even better (the window in your microwave oven, for instance).
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