Re: [Amps] insulation

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

John T. M. Lyles wrote:
> G10 will melt down into an awful stinkin' and burnin' mess when heated 
> in high Rf fields. I rarely use it anymore, except for PW applications. 
> It is certainly a big step above Delrin acetals or nylons. Have settled 
> on several wonder materials like:
> Rexolite (crosslinked polystyrene) rod and sheet
> Polyetherimide (ULTEM* 2300) with 30% glass, also known as Tempalux*
> Polysulfone (UDEL*), also known as Thermalux*
> G7  (silicone resin reinforced glass)
>
> All more expensive, but if you want the highest Q and no heating, they 
> are worth it. These are all high temperature engineered plastics. If 
> you are concentrating E field flux in the dielectric, even with a kW, 
> G10 will heat up. Good old UHMW (ulta high molecular weight) 
> polypropylene and polyethyline made good insulators but have low glass 
> transisition temperatures and will soften and dimensionally change with >heat.
>
> Your local plastics supplier has them. Pricewise, the first two are 
> about $1000 for a square foot of 1 inch thick material! The rod stock 
> is much cheaper, and for smaller coils it is more so. The G7 is more 
> difficult to machine or turn on a lathe than G10 due to the lamination 
> layers. But it is excellent material structurally as well as RF'ly.
>
> Cheers
> John
> K5PRO
>
> Rich AG6K said:
> >  I bought 50-lbs of porcelain clay and I tried making porcelain
> > pottery.  The shrinkage factor is about double ordinary stoneware clay,
> > and porcelain clay is hellish to work with.  My advice is use G-10
> > fiberglass-epoxy for insulating.  It's good around RF, strong, and easily
> > drilled/machined.?
> >
> > > My YL has a ceramics business, and I started thinking about trying to
> >  >manufacture my own ceramic (porcelain) insulators. Has anyone tried this?

The difference between John and the rest of us is that all John's RF 
fields are guaranteed to be "high"!

For the rest of us, even in our QRO amps there are many locations where 
RF fields are actually quite low... but also places in "driver-power" 
gear where the fields can be high enough to surprise us.

I read here that Delrin pillars are used to support the tank coils in 
some Alpha amps. If Delrin is OK for that application, the reason must 
be because there actually isn't enough RF field inside the pillars to 
cause significant heating. And once you know it's going to be OK, Delrin 
becomes an excellent choice because it's so nice to machine and 
fabricate.

The opposite example is that I once used Delrin for the PA tuning shaft 
in a little 6522 2m transmitter. The capacitor rotor was *supposed* to 
be at RF ground, and that TX couldn't have been putting out more than 
25W, tops - but the Delrin melted in seconds. Wrong place, I guess.

Coming back to ceramics, some grades are not especially low-loss, and 
suffer more RF heating than many modern plastics - more even than Delrin 
and the equally despised PVC. Ceramics get away with it because they can 
also handle high temperatures. At one time, ceramics were about the 
*only* materials with that particular combination of properties... but 
that isn't true any more.

Another material that's often overlooked is glass. If you know of a 
technical glass shop that carries tubing stock (which will be Pyrex) you 
have a very good source of choke formers, mounting pillars and even 
custom tube chimneys.

For formers and pillars, simply cut the tubing to length and epoxy a 
brass nut into one or both ends... which once again shows that even a 
mediocre RF material like epoxy can be used, if it's not subjected to 
large field gradients through itself.



--
73 from Ian G3SEK
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