Several things could explain what is described:
1) The RF oscillator in the preheater is free-running. It is affected by the
dielectric in the air capacitor zone. In addition, it is coupled such that as
it is loaded, voltage is not constant, as the frequency is pulled. I measured
the voltage years ago with a spark gap using 1 inch balls without loading. Lot
of variability there. To do it right I would have had to breakdown a ball gap
in each measurement, which is very time consuming. So I approximated 15 kV as
typical voltage, and it is certainly a very strong RF field such as might be
expected in a large amplifier or resonator.
2) The RF current that is referred to from calculated reactance and voltage is
circulating in the tank network of the heater. I was using DC plate current
from the plate ammeter.
Someone pointed out to me, in asking why I could not measure favorite material
XXX or YYYY and post that too. While I would love to have the time and variety
of prepared pucks of each material that we all may use in our craft, I was only
concerned with the reliable and known materials that I have refined in my own
toolbox, for RF insulation. YMMV - as they say! I cannot afford to be using
plastic materials that are not specified for dielectric properties, marginal or
absorb moisture in high fields. It is just a waste of my own time if I know it
might bite me someday, or melt down in the middle of the night in machines that
run 24/7. You can buy the same materials at any plastic suppliers worldwide, or
use a substitute with less-known characteristics. It pays to at least heed the
dielectric data on the material data sheets, except that many times it is only
specified at 1 MHz or 100 KHz, and not at HF or VHF. Loss tangent varies in
most cases. That said, I would be happy to du
plicate
the experiment in a more relaxed time frame, if sample pucks were provided to
me in the correct geometry for comparison. This might lead to some interesting
if not ugly postings! RF is such a fickle thing, when we are dealing with
standing waves and voltage/currents in the wrong places and materials.
73,
John K5PRO
> Message: 3
> Date: Tue, 28 Aug 2012 09:23:57 -0600
> From: Larry Benko <xxw0qe@comcast.net>
> Subject: Re: [Amps] RF insulating materials - engineered plastics
> To: amps@contesting.com
> Message-ID: <503CE28D.7050803@comcast.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> 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
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