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Re: Overcoupling?

At 12:30 PM 5/1/99 +0100, you wrote:
>Dear All
>
>I was tuning up my coil again a couple of days back (I now have my other
>NST back from repair and have stretched by alu duct toroid to 36" dia - 2kW
>in) and at full power (2kVA) it now tunes at around 8.3 turns (WinTesla
>predicted 7.6). However when I hit this tune I think that I identified
>symptoms of 'double-humping' - I saw a ring of heavy inter-turn corona and
>mild sparking about 3/4 of the way up the secondary, and the occasional
>spark all the way down the secondary coil.
>
>I raised the secondary up about 1/2" and this was reduced, although not
>absent. The sec base is now about 1/4" above the level of the primary, and
>the gap around the sec is about 1.5". My toroid's centre disc is temporary,
>not covered completely with foil, so maybe this is causing probs too.
>
>Anyone else have any experience with coupling adjustments? If you have any
>advice I'd be happy - I don't want to reduce the coupling too much...
>
>PS Spark length so far is 60", although since I am in a confined space, if
>I try for beyond this, the coil seems to favour the roof beams and walls as
>targets. I hope I can achieve more - do you think I can get that 6 feet??
>
>Alex Crow

Hi Alex,

	When in doubt, I would reduce the coupling.  Too little coupling and your
sparks are short. Too much and your coil may burnup!!

	The corona area you mention is probably a natural high field stress area
that linear wound Tesla coils have about 2/3 up the secondary.  Raising or
lowering the top terminal may help this if it looks like it is going to
arc.  I would fill in the center disk covering as this may help the high
field stress things you are seeing.  I think your coupling is really too
high and the output sparks may easily grow longer if you reduce it.
Overcoupling is really bad and can cause many problems.  It is always a bit
odd to move the secondary away from the primary to get longer sparks but
this is often the case...

	Terry