On 12/6/13 12:53 PM, Steve Ward wrote:
Hi Jim, Just a few comments/questions.bear in mind that tesla coils run at very low frequencies compared to HF radio. Folks who come into this from the ham radio side get all excited about "that long ground wire", because of their experience with HF antennas .. but that long wire is a tiny fraction of a wavelength at 100kHz (lambda = 3km).Most experiences with EMI and TCs (at least for solid-state driven types) suggests that the fundamental frequency is not where the trouble is. That is, when the TC is just sparking into free air there usually isnt a problem, but when the sparks find a large charge-accepting body (not even necessarily ground, could be a large isolated object) then higher frequencies are generated. I measure big spikes in the 5-10MHZ range, but also the entire spectrum above that is elevated, so 10s of MHz easily... Any ideas on dealing with that?
I think that's more like the EMI from lightning.. you've got a pulse with a rise time in the "few tens of ns" range and that has strong spectral components there.
The question would be: is the EMI getting to the victim by radiation or conduction? If it's by conduction, then filtering "wires" that lead to/from the coil would help, and for 1 MHz on up, conventional powdered metal/ferrite cores are probably the right way to go. I use a lot of 2.4" diameter toroids in the 31 mix from Fair-rite. It has good resistance and inductance over a pretty broad band in HF, unlike the ones you see used on computer cables, which tend to be optimized for higher frequencies.
http://www.audiosystemsgroup.com/RFI-Ham.pdf is a useful reference on conducted and radiated emissions, with lots of data for filters using magnetic materials.
Terry Fritz found significant emissions in the VHF range for spark gap coils. For that, it's probably radiating from the wires going to the gap.
And what you really don't want to do is have your coil inside, and ONLY grounded to a stake in the ground outside. Now the RF path *is* going through the building.I believe that anything near a TC becomes part of its "RF" circuit just because of mutual capacitance with the TC itself. So being near the very high dv/dt of a ground-sparking TC can induce currents in un-intended paths, and your grounding system may not make enough difference to avoid this problem. An electrostatic shield (not truly a faraday cage) often fixed this problem. Sometimes your ground plane should go vertical so as to capture more of the TCs field.
Yes. I did a back of the envelope analysis of this at http://home.earthlink.net/~jimlux/hv/tcemi.htm
The real issue is whether the radiated problem is E field or H field.
Steve _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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