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Re: More facts: Coil size to faraday cage size ratio

Original poster: "Mike" <induction@xxxxxxxxxxx>

Hi,
I totally agree with the ferrite cores, the line filters, etc. We went through a horror story.
At an AM and FM combo radio station I was the CE of, 5 kW AM, 3 kW FM, all was normal.
Usual once in a while getting into telephones, Ferrites and filters usually fixed it.
Then across the street from that site, a doctor set up an office with a long term lease he could not
get out of. He specialized in nerve and muscle problems. Three treatment rooms all ended up
equipped with EMG machines, scope, meter and audio outputs. After all the renovations and when he was
actually beginning to TRY to work there, he found he had three of the most expensive radio receivers
ever purchased! I got the call and went out to see. First, the pole pig was between him and us, the power lines
from it ran in almost a straight line to his building referenced to tower one. RF was dumping to his lines. He was also in the near field. Removing the hung ceiling tiles here and there showed he had romex, commercial property it should have been armored anyway. Filtering at the panel was out of the question, the whole wiring system was still going to be antenna.
We ended up getting very thin copper sheet, more like conductive wall paper, worked with the guys contractor. Each of the three treatment rooms got the same, heavy filtering of all power lines at the wall entry point of each room, the telephones, etc.
the paneling was removed, we added and bonded the sheets, used 6 inch wide strapping to carry the walls to ground and rods we drove in a chipped out area of the concrete floor, later filled in. The doors had sheeting added and this was later covered. We made copper flex "hinges" at 6 points of the hinged side of the door while the standard hinges took the load of the door, ours were to keep the door connected to the walls and ceiling of the shielded room. All inside rooms, there were no windows so that was good. When we were done, the only place you could tell it was shielded was the hinge area of the doors. It worked great. With the door closed in each room, the signal left his equipment with no sign. I took the field strength meter and got a reading equal to being out about ten miles when we did proofs.
If we had not filtered all the lines at the entry point to these rooms, I think the leakage would have been too much. I was also grateful that there was a couple of interns to help out. The doctor was very happy even though he had paid for all the material and contractor cost The station covered our labor as staff.
All in all, we got lucky.
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, November 23, 2004 8:58 PM
Subject: Re: More facts: Coil size to faraday cage size ratio


Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

I also second the use of big ferrite cores liberally placed around the primary and especially at the spark gap.

Right before the spark gap fires, there is a lot of energy stored as capacitance between the gap electrodes. When the electrodes fire, that energy is shorted out without much R and L in the way which causes giant RFI.

All my spark gap coils have ferrites but I never think about them much since they are passive things. Note my big gap that has ferrites directly on the electrodes:

http://hot-streamer.com/TeslaCoils/MyCoils/BigCoil/Rgap.jpg

I can't really quantify how much they help, but they do reduce RFI "a bunch".

DigiKey sells them from very small to pretty big:

http://dkc3.digikey.com/PDF/T043/0793.pdf

Maybe you just need to cage the spark gap.

Also, streamer just to air will have far less RFI than streamers striking to ground.

Hope this helps!

Cheers,

        Terry


At 11:53 AM 11/23/2004, you wrote: 
Since the signals of concern are fairly HF (i.e. >30 MHz), it would be
interesting to find out where they are coming from.  Is it high order
harmonics from the LC ringdown (unlikely... we're talking VHF frequencies
here, and 100 MHz would be 1000th harmonics of the 100kHz fundamental).
Probably more likely radiation from primary circuits (as Terry had noted
some years back) or from the streamers themselves.

So, maybe something to kill the HF content in the primary circuit (ferrite
cores on the wires to/from the sparkgap?  Have to choose an appropriate
material, so they don't absorb too much energy at the fundamental and
overheat.
----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, November 23, 2004 9:00 AM
Subject: Re: More facts: Coil size to faraday cage size ratio


> Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>
>
> Hi Fin
>
> Its probably cheaper and less demanding on the each cage to have a > double
> one.
> Two poor performance cages can be more effective than one good one and
> possibly two are essential if the inner is going to be in the discharge
> path