Re: wood core coils

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


und.
> 
> > Use white PVC
> > plastic pipe to avoid
> > trouble that is common to wood and cardboard.
> >     Robert H
> 
> Something that has been bothering me. I've seen people
> refer to PVC as being able to absorb water, and
> suggest baking it to drive the water out and then
> giving it coats inside and out of sealant
> (polyurethane or laquer, clear only). 

Sure, if you want to obssess... It probably doesn't change the breakdown
strength all that much (unless the pipe has been laying in a puddle...).
Cleanliness and good HV construction technique is probably more important.
No sharp edges, single wires out by themselves, etc.

I also recall
> long ago seeing an article in the ARRL magazine where
> someone had used PVC to make loading coils for a
> multiband dipole antenna, and when he keyed up a KW on
> it the coils failed by arcing a path across the PVC.

Yep.. and how did he make his coils?  Recall this is at substantially
higher frequencies, so standing waves and resonances might result in a very
weird voltage distribution.  Was the antenna sitting outside with moisture
creeping in under the varnish? What about air gaps or dust?

> He had a photo of the PVC cores with the wire removed,
> and there was the classic branching pattern burned
> into the PVC.
> 
> So does PVC really able to absorb moisture? Perhaps
> this guys antenna cores failed because he didn't seal
> them.

 Has anyone tried ABS black pipe, or does the
> black come from carbon for color? 

Yep, carbon black for color and UV resistance.  More to the point, check
the label on that ABS DWV (Drain Waste Vent) pipe and you'll see the words
of evil: cellular or foam... The reason that ABS pipe is so lightweight and
cheap is that it is actually a clever sandwich of solid abs on inner and
outer walls over abs foam. 

>From a HV insulation standpoint, uniform materials are best, and foams are
positively horrible (the little bubbles cause field concentrations).


> 
> My original intention was actually for a Van de Graf
> generator, but this got me to thinking about Tesla
> coil cores.

PVC has problems with VDG (although mine uses PVC) because the moisture
does increase the leakage (by reducing the resistivity) which is a big
problem with static electricity machines.  I don't think that the breakdown
is the dominant problem with the VDG (it just arcs over the surface, which
would be the case with ANY insulator).  A TC is a much higher powered
device, so the resistivity is less of an issue.

>