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Resonators in enclosed spaces

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Steve Conner

Sat Jul 14 2007, 09:25PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, for the sake of good neighbourliness, namely not pumping several MHz RF current into the mains ground, I'd only consider options B and C.

Ideally, I'd use a full Faraday cage instead of just a sheet, and use option C. If you only have one side of the "cage" in place, then RF current will still couple into the surroundings, and hence a corresponding current will flow back to your setup through the mains ground wire to complete the circuit.

Provided the cage is made of a material that conducts well and is non-magnetic, and is big enough to leave space round the coil as we discussed, a full Faraday cage should actually give better performance than no cage. I've heard rumours that one UK coiler managed to make a 144MHz Tesla coil by enclosing a quarter wavelength whip antenna in a Faraday cage, and driving it with a high-powered transmitter. Without the cage, the antenna could never break out as it just radiates all the power!

That's what I visualise anyway, YMMV.

ShawnLG

Sun Jul 15 2007, 05:27PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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Posts: 399

I have been planning on breaking BP record. I will not give any information on this until the record is broken. Steve is right, use a complete enclosed Faraday cage. I would suggest grounding the secondary to the cage, because the Faraday cage would be part of your LC circuit. RF leakage should be minimal with this setup.

ragnar

Mon Jul 16 2007, 12:10AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

What record?

Ken M.

Mon Jul 16 2007, 12:21AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I think he means your high frequency, I think its 13MHz?

ShawnLG

Mon Jul 16 2007, 06:00AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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Posts: 399

"What record?"
Your 13.56Mhz CW SSTC.

ragnar

Mon Jul 16 2007, 09:33AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Yes, doing that with a $1 MOSFET was interesting. I'm afraid I do also have a smaller 17MHz SSTC, however. =)

WaveRider

Mon Jul 16 2007, 07:56PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Just a thought: When a CW tesla coil is in breakout mode, the Q is determined by the loading of the plasma discharge. In my coil I have estimated this to be of the order 10-30.

When the coil is not breaking out, the unloaded Q of the coil can be of order 1000. Loading caused by the sorce reduces this to 100-500.

The efficiency of the coil will be proportional to the ratio
Ql/(Qu + Ql). If Ql (arc-loaded Q) is << Qu (Q without breakout), then small changes in unloaded Q by introducing various shielding schemes will have little effect.

And, as Steve mentioned earlier, using a shield greatly reduces inadvertent coupling into external "antennas" in your couch or house wiring... Radiation and proximity coupling are worse drainers or spark power than you might imagine in HF coils...

ragnar

Tue Jul 17 2007, 02:17AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

I admit I have often underestimated the radiation loss / couch coupling factor. -- That confirms why my SMPS's all click.

From my original problem:
Let me give an example:

If I have a very small secondary (1" high, 1" in diameter), and place it on a square-foot sheet of copper clad, breakout is next to impossible.

In this case, my best bet is to ELEVATE the secondary, perhaps a few inches away from its copper groundplane, and still tie the copper to mains ground?

Steve Conner

Tue Jul 17 2007, 10:12AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes.

Marko

Tue Jul 17 2007, 04:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

joe wrote ...
No f**** way! I don't have much understanding of RF and magnetics, but a conductive shield increasing coupling is just counter to all my beliefs! While the shield will in a certain sense "reflect" the electromagnetic waves impinging on it, it will do it in such a way as to cancel out the impinging field. Unless there is a funny phaseshift somewhere, the field reaching the receiving coil should be made smaller, not bigger.

Another demonstration of how magnetic field reflects.

When a conductive plate is positioned about 45 degrees between coils bulbs on receiving resonator glow significantly brighter. Quite similar to what we would expect to see with a light beam.

Interestingly, I found most non-magnetic materials, including brass and even non-magnetic stainless steel to work about equally well.

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