Re: Secondary form materials

• To: tesla-at-pupman-dot-com
• Subject: Re: Secondary form materials
• From: "Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Fri, 17 Mar 2000 07:50:15 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
> 
> Original Poster: "Unger, Blair" <ungerb-at-william.jewell.edu>
> 
> I'm beginning the research and sometime down the road, construction of a
> Tesla coil whose secondary will be approximately 1.5 meters high and .15
> meters in radius. I need some help clarifying what material to use for the
> secondary coil form.
> 
> In the August 1997 edition of the "American Journal of Physics," I came
> across an article that stated the only sensible way to create the secondary
> coil was to have as little dielectric material in the vicinity as possible.
> In the article, this was accomplished by spacing six small acrylic tubes at
> the edges of a hexagon, leaving the center open to the air. Is it necessary
> to go to such an extreme to limit dielectric loss. If not, what material
> would be best suited for a secondary form.
> 
> Thanks,
> 
> Blair

Blair,

It is definately not necessary to go to such extremes, and in fact using
an open coil structure may actually limit your coil's maximum output due
to flashover problems. Coilform RF dielectric losses are actually a very
minor concern as long as you use a good HV insulator such as PVC, PMMA,
Polycarbonate, LDPE, HDPE, or PP. PVC is one of the better coilform
materials, being readily available and inexpensive. Simple thin-wall
schedule 10 or SDR PVC pipe works great. Using thin-wall PVC pipe helps
keep the weight down, but thick wall schedule 20, 40 or even 80 PVC pipe
will also work virtually as well. PVC's high resistivity and high
voltage breakdown properties also help prevent the internal arcing that
might otherwise be a problem for an open-wound coil. 

PVC is indeed lossy at RF frequencies, and it does reduce the
instrumented Q of the secondary a bit. However, this is not worth
worrying about from a practical standpoint. Once streamers form, their
loading effect reduces the effective secondary Q to a much greater
extent. For high performance coils, the insulating and voltage standoff
attributes of PVC far outweigh the small decrease in Q. Save your time
and money - go for PVC! :^)

Safe coiling to you!

-- Bert --

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