>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.
? Ian: I did not and do not suggest that heat from contact with the
wire causes any of the Delrin bubbling observed by users of MFJ tuners.
>In the end, the only way to find out is to try it.
? I found out that Nylon, PVC, and Delrin had a Dissipation-factor
problem by reading the specs. on them at the local plastics supply house.
> 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.
? There are many applications where Delrin would be better than PTFE or
ceramic. RF-applications is not one of them. Much hay has been made
over how hot the roller coil conductor gets. "Curious minds want to
know". The 25uH roller-coil in my antenna tuner becomes warm to the
touch of my thumb during use (key not closed during temp. measuring
segment) . . The roller-coil uses #14 silver wire and a ceramic coil
form. The roller-coil was made by GE in 1943. The amp. is a pair of
- cheers, Ian
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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