Re: secondary forms
Subject: Re: secondary forms
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Date: Thu, 8 Feb 1996 08:53:34 +1200
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> I see that RF wise PVC is not so good as a secondary coil form (I myself
> have been using sonotube at first then PVC). Could you use a long cylinder
> with a layer of Mylar or Poly on it, then wind the coil over this, coat it
> with epoxy (Polyureathane), let dry, remove the long cylinder (and possibly
> the Mylar or Poly), and then coat the inside with more epoxy. Now all that
> should be left is a coil held togeather with epoxy, no other form, no PVC. If
> there was enough epoxy it seems like it would hold togeather. Please let me
> know the + and - of this idea, I am sure someone has thought of this already,
> but I am curious.
This is more-or-less exactly what I'm planning to do for my 12" coil.
I checked the Q of a coil I'd wound on the sonotube (which hadn't been
all that well prepared) as far too low and dependent on air moisture
content. I'll be using varnish rather than epoxy and I'm planning on
retaining the polyethylene sonotube wrap although my plans are still
somewhat hazy on this. A free-standing coil is an attractive
proposition but raises some interesting problems when it comes to
handling strength, mounting terminals etc.
BTW, I'm keen to get a large toroid commercially spun to get
a nice smooth surface and also to go for a large ring cross-section.
I talked to a couple of aluminium spinners here and it seems that
the bulk of the cost is in making the wooden formers, so I'm cur-
rently hunting down a wood turning firm with a large lathe.
And in connection with terminals, there's an idea I want to
explore that involves mounting two moderate sized toroids, one
above the other to form a larger "virtual" toroid. I don't know
what the outcome will be but I intend to try it to compare volt-
age holdoff with a single large terminal.
Any thoughts on these is most welcome.