Minor correction - you used diameter of the puck instead of the radius when
calculating the area.
C = 2.1 * 0.2248 * (1.5/2)^2 * pi / 0.75 = 1.11 pF
Xc = 1593
I = 10607/1593 = 6.66 Arms
--- Jeff WN3A
> -----Original Message-----
> From: amps-bounces@contesting.com
> [mailto:amps-bounces@contesting.com] On Behalf Of Larry Benko
> Sent: Tuesday, August 28, 2012 9:51 AM
> To: amps@contesting.com
> Subject: Re: [Amps] RF insulating materials - engineered plastics
>
> 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
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
>
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