John,
This is extremely interesting but something seems incorrect.
Assuming your teflon dielectric puck:
C(pf) = .2248 * K * A * (N-1) / d = .2248 * 2.1 * PI * (1.5^2) * 1 / .75
= 4.45pF
Xc @ 90MHz = 1 / (2 * PI * 90e6 * 4.45e-12) = 397.4 ohms
15kVpk = 10607Vrms
I = 10607 / 397.4 = 26.7A which is over 100 times more current than you
show.
Can you explain the discrepancies?
73,
Larry, W0QE
On 8/28/2012 12:40 AM, John Lyles wrote:
> 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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