[Amps] more on RF insulation

[John T. M. Lyles]

Comments have mentioned Delrin*, nylon and Teflon*:

Delrin* acetal resin is a crystalline thermoplastic made from 
polymerization of formaldehyde. In the handbook by Dupont, it has 
lots of outstanding properties, but for electrical it is listed as 
'good' properties. I know it is used in a lot of AC mains rated 
insulation, motors, applicances, etc. Its dielectric strength is 
good, but that is only a DC/low freq measurement. By the way, Formvar 
insulation is an acetal also, but it is such a thin film on magnet 
wire that it probably has little effect. The dielectric constant 
varies significantly with temperature. The loss tangent (dissipation 
factor part of complex dielectric property) is dependent on moisture, 
and it is somewhat hydroscopic. As a matter of fact, it doubles for a 
moisture content increase from 0.2 to about 0.7%. It runs from 0.005 
at 1 KHz to 0.0085 at 0.5 MHz all at 23 deg C, but at 1 KHz and 125 
deg C it is as high as 0.012, which isn't considered and RF insulator 
of much quality. Its between bakelite and plexiglass on the charts, 
and sort of similar to the resin in G10 fiberglass material. But more 
unstable as a dielectric.

Teflon* is very stable as an RF insulator, as TFE loss tangent is 
<0.0001 from audio to 1 MHz, and then rises to 0.0045 at high UHF 
frequency at 25 deg C. Teflon* FEP loss tangent is only 0.0012 at 
above a GHz at 25 deg C. The loss tangent actually drops with 
temperature, which is why it doesn't go into thermal runaway when put 
in high RF fields. The RF properties don't change significantly (like 
in Delrin*) with humidity either. If it weren't for the cold flow 
problems, Telflon* would be the best RF insulator known in my 
opinion. Whenever there is a strength issue, however, it fails. For 
coil forms it would do fine however, i neglected to mention that 
before.

I don't have much info on nylon's properties except in fiber form, 
and very old stuff from Dupont. However, it has noticable properties 
like Delrin* in that it gets more lossy with temperature and with 
moisture ingress. When I worked for Dupont (1985-1991) I did a lot of 
work RF heating nylon resin for various industrial processes, and 
after that experience I would not consider it a good Rf insulator 
either. BTW, it really stinks when it gets hot! And virgin nylon 
isn't very strong mechanically, unless it has additives like glass.

For a good source of this sort of info, find a copy of Von Hipples' 
"Dielectric Materials and Properties" from MIT, in reprint now, or 
one of the MIT Rad Lab series texts on microwave insulators. They 
measured a lot of stuff, particularly at 3 GHz and above.

73
John
K5PRO



*Teflon and Delrin are Dupoint tradenames.