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[AMPS] coil form plastics

To: <>
Subject: [AMPS] coil form plastics
From: (John T. M. Lyles)
Date: Fri, 12 Feb 1999 13:07:59 -0700
To: <>
>Date: Thu, 11 Feb 1999 01:37:49 -0800
>From: Rich Measures <>
>Subject: Re: [AMPS] Trade roller(s) for roller

>>I would like to trade both of these units for a Johnson 229-203 roller coil.
>>I have a homebrew copy of the MFJ 986 differential-T tuner and the coil that
>>was used is TERRIBLE. I think it is on a Delrin core and with higher power
>>levels the impedance starts to change which effects the tuning of my hf
>>amps. Yuk!!

>.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?

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. I needed strong
physical properties, as well as low dielectric loss. PTFE had unacceptable
cold flow as well as near inability to survive strong ionizing radiation
such as gamma.  When I worked on RF heating for industrial uses at a
*large* chemical company in Delaware, we used G7 to support the electrodes
in giant dielectric heaters. Nothing else would hold up. It is obnoxious to
work with, like G10, but not as well 'epoxied'.

G10, FR4, and other glass epoxies are OK, but the resin is phenolic-based I
believe and it absorbs RF, therefore slowing heating in the field. RF
heating is proportional to E^2 x F, so you can see that it has a stong
function dependent on voltage. I threw chunks of each material and also
some various water hoses, into my old WT LaRose Thermall RF preheater, it
runs about 90 MHz with maybe 15 KV field. Only a few minutes will tell the
difference. What a stinkin' mess the G10 was.  G7 has much better RF
characteristics, but, like Udel*, it's quite expensive. You can see both in
the photo below, supporting the plate RF choke. The amber colored form is
Udel* and the white colored standoff brackets are G7. We were unable to
turn coil grooves into the G7 rod, as it delaminated in the lathe. Hence
the change to Udel* for that. It was very nice stuff, and I bought a stick
of it from a local plastic distributor. I have seen Polysulfone in some
high power decks before. Another great material is Rexolite (crosslinked
Polystyrene). It make wonderful waveguide windows. Also it makes nice
insulators in cavities. But is is very brittle, and tough to machine. The
dielectric properties are not far from PTFE, lightly higher K, and loss is
maybe 10x, still negligable even to X band.



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