I just finished a series of comparison tests of a handful of polymeric
materials and blends, for use in high power VHF amplifier projects. I
didn't test Nylon, Delrin acetal, Polyethylene or Polypropylene. What I
did test was vigin PTFE, 25% glass + PTFE, Polystyrene (Rexolite),
Polysulfone (Udel), Polyetherimide + Glass (ULTEM) and G7 (glass +
silicone resin laminate). One new material that i was skeptical about
was Duraform, a material used in 3D solid printing. All are materials
that I have used one time or another in high power projects in my
workplace. Each material was machined in the same shape, 1.5 inch
diameter cylindrical puck, 0.75 inches thick. They were placed in a WT
LaRose "Thermall" Model 9 dielectric preheater, using a single Amperex
5868 triode at ~90 MHz. Approximate voltage across parallel plates was
15 kV peak, and the plates were compressed against each sample, without
air gaps. As they were inserted, all materials loaded the machine at
roughly the same plate current. Temperature was taken with a Stanley IR
thermometer, the point and shoot type, at 6 inches from flat surface of
puck. By soaking them in this extreme field for 2 minutes, I was able to
make a realistic relative comparison of their loss tangent at 90 MHz.
This is worthhile information when designing with polymer insulators
inside amplifiers.
Now the results, sorted in order of loss:
Material StartTemp EndTemp Start Ip End Ip Comment
PTFE (Teflon) 84 degF 86 deg 210 mA 210 best
PTFE+Glass 84 98 20 205 goodmechanical
Polysulfone(UDEL) 86 188 200 200 translucent amber
PEI (ULTEM)+Glass 84 207 200 203 laminate
Polystyrene(Rexolite) 84 215 210 >300 runaway, 45 sec
G7 84 292 190 190
Duraform 84 210 200 >350 runaway, melt 10 sec
Conclusion: Virgin Teflon is the winner, as expected. If it weren't for
the difficulty of mechanical stability and strength, there would be
nothing else needed. But it is quite costly now, since the Japanese
disaster. A good substitute for strength is the Teflon+Glass mixture.
UDEL and ULTEM are both great materials, and about similar in loss
properties as medium quality dielectrics. Rexolite, widely used for
radomes, coaxial line supports, and in cavities, has a runaway loss
tangent property with temperature as it nears glass transition.
G7 is good for lower frequencyies, not so at VHF. Duraform is a terrible
insulator, but great as a printable material for 3D models.
I welcome any comments or suggestions.
73
K5PRO
John
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