|Subject:||[Amps] more on RF insulation|
|From:||"John T. M. Lyles" <firstname.lastname@example.org>|
|Date:||Fri, 26 Mar 2004 10:52:25 -0700|
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. _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps
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