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[AMPS] coil form plastics

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Subject: [AMPS] coil form plastics
From: (Ian White, G3SEK)
Date: Sat, 13 Feb 1999 09:31:07 +0000
John T. M. Lyles wrote:

Rich wrote:
>>.Not only that, Lane, a Delrin core will assuredly melt and bubble at the
>>kw level, as the ARRL Lab discovered with two MFJ "3kW" tuners, and
>>apparently failed to report in *QST*.    It seems that the person
>>(Elliot?) who designed this roller-coil was clueless about the RF
>>properties of plastics.  ABS, G-10, or G-11 would have worked ok, Delrin,
>>no way.    .  .

>Delrin is an acetyl resin, does make a nice 60 Hz insulator, thats about
>it. And it is quite hydroscopic to boot!
>One would be asking for big trouble to use it over a few KV of RF voltage
>at high frequencies I think. As you pointed out above....... how do people
>get away with using it?
Probably by not using it in situations of high RF stress (voltage
gradient) or high temperature. I've managed to melt Delrin by putting it
in a particularly stupid place in a 144MHz PA running only 20W!

Roller coils seem to be a particularly bad situation, where the voltage
gradient can be very high, and also the material can be heated by losses
in the coil. In addition, it's very easy to adjust a T-network tuner in
a way that gives more losses than need be.

>The following information may be overkill for low duty operation at the 1-2
>KW levels. I present it for information, so that you can see what advanced
>dielectrics are capable of, should you want to pursue the levels of
>performance similar to what I needed recently.
>I needed supports for chokes and components for Rf voltages of about 20 KV
>peak at a few MHz. Udel* (polysulfone) and silicone-glass laminate G7 work
>well over the entire HF and VHF ranges, except in extreme high gradients
>where PTFE (Teflon* PTE, FEP, etc) would be recommended.

Thanks for the excellent information, John. However, substitutes for
real Teflon (PTFE) may not always work. 

One guy in DL had a lot of trouble with a 1.5kW PA on 432, which tuned
up but produced almost no RF output. It turned out that there was plenty
of RF being generated, but he was coupling most of it into the big,
thick cooling chimney. He believed it was PTFE (dammit, he paid enough
for it!) and it looked and felt like PTFE - but a test with a cigarette
lighter showed that it wasn't. 

It was one of those 'near-PTFE' polymers like FEP or something similar,
and it charred and burned where real PTFE would only have softened and
turned yellowish at those temperatures. When he changed the chimney to
real PTFE, the whole amp worked just fine.

Sometimes the problem with plastics like Delrin is that they can't stand
the temperature, and losses seem to increase dramatically as the
material softens so you get a runaway situation. If mechanical rigidity
is important, it may be better to use something like Pyrex glass and
accept that it will get a bit warm.

On a possibly related subject, does anyone know a source of sticky-
backed Kapton tape?

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

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