Topband: Insulator problems

Tom W8JI w8ji at w8ji.com
Sat Dec 14 06:05:56 EST 2013


> 2)  I always thought that nylon was a pretty good dielectric, and did not 
> expect problems, especially at 1.8 MHz.  The gap in the insulator is 
> 7.5mm, or about 0.3".  I estimate that there will be around 2 kV across 
> this gap.  Is nylon perhaps not as good as I thought it was?

Nylon is one of the worse dielectrics for RF. This shows when the electric 
field density is high, so it does not mean nylon won't work in some 
applications. For example, a nylon spacer in a typical linear amp with a 
screw protruding down in it will sometimes bubble and smell around the screw 
on the vacuum tube side of a tank circuit, but will be perfectly fine on the 
50 ohm side where voltage is low.

The 1 MHz dissipation factors are around:

nylon = .022 or worse
Delrin = most types .005
Teflon = less than .0002


> 3)  If I replace the nylon with Teflon, will I lose anything in mechanical 
> strength?

Teflon is soft, but wear resistant and reasonable with compression loads. It 
has the least tendency to carbon track. Delrin is the best mechanically, 
unless it does not have to be rigid.

I would think either Delrin or Teflon would be OK electrically, if you got 
years of service out of nylon. 



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