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[AMPS] coil form plastics

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Subject: [AMPS] coil form plastics
From: (Ian White, G3SEK)
Date: Sun, 14 Feb 1999 11:28:09 +0000
Richard W. Ehrhorn wrote:

>For what it's worth, my experience is that Delrin is an excellent rf 
>insulator, with very low loss at least in the HF range. It has reasonable 
>mechanical stability (like nylon and unlike teflon) and machineability. We 
>successfully used Delrin tank coil supports in thousands of Alpha amps.
>Of thousands of amps using Delrin coil supports, we had only a couple of 
>Delrin ignitions, and those clearly were caused by rf arcs NOT attributable 
>to the Delrin. I know of no case where Delrin was ignited by self-heating 
>due to dielectric loss. In my judgment it's a practical and useful material 
>for HF applications as long as ambient temperature (including heat from 
>windings it contacts, etc.) is below ~100 deg. C.
I think Dick's examples illustrate the point very well - it's much more
about *where* you use it than the material you choose.

>Ian, your point about losses in Delrin perhaps increasing rapidly with 
>temperature is one I hadn't thought of and bears investigation. Do you have 
>any relevant data or experience for a starting point?

Unfortunately it's not much more than an intuition based on my physical
chemistry background, that the coupling of any material to an RF field
will become stronger as it softens and the molecules become more mobile.
If the effect exists, it would be related to closeness to the softening/
melting point, rather than simple temperature.

Maybe it's just that low-melting materials such as Delrin, polystyrene
and polyethylene are very unforgiving. If either the designer or the
user makes a mistake, it will fail spectacularly. Higher-temperature
materials such as ceramics will just get hot and carry on working.

Coming back to the roller coils, Rich makes a good point that *if* the
material has bubbled internally, that suggests internal RF losses as
well as external heating by contact with hot metal. 

In the end, the only way to find out is to try it. For situations where
an inexpensive and easily machined material such as Delrin is proved to
work, there's no point in paying for PTFE or ceramic.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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