[Amps] RF insulating materials - engineered plastics

[John Lyles]

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 duplicate
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 at comcast.net>
> Subject: Re: [Amps] RF insulating materials - engineered plastics
> To: amps at contesting.com
> Message-ID: <503CE28D.7050803 at 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