Jeff,
Whoops! You are are correct and thanks for finding the error. That
makes the current amplitute discrepancy 4 times less but the calculated
current is still 32 times the reported current. Will wait till John
reads this for hopefully an explanation.
Larry, W0QE
On 8/28/2012 9:18 AM, Jeff DePolo wrote:
> 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
>>> _______________________________________________
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>>> Amps@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
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