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[AMPS] coil form plastics

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Subject: [AMPS] coil form plastics
From: (Rich Measures)
Date: Sun, 14 Feb 1999 06:35:24 -0800

>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,  

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