On Wed, 17 Feb 1999 15:45:15 -0700 "Richard W. Ehrhorn" <email@example.com>
>It's only a gut feeling, but I'd prefer a single thicker piece of
>multiple thin ones, because the latter creates more interstices (!?!)
>between layers - where air can lie in wait for the chance to getcha.
OK Dick BUT that was why I asked the question. Unless you buy the top
grade Teflon sheet there will be air voids...period.
So why not isolate those voids with additional sheets under the
"assumption" that no 2 major voids will line up.
Throw some silicone grease in between for good luck and voila......just
maybe a superior cap ? I dont know, so just throwing the idea out for
discussion. God knows we need few new subjects here.
>Think John mentioned rounding/radiusing the edges of he electrodes to
>sharp edges AND maybe using some sort of (low-loss, obviously)
>paste at all interfaces and especially in the tapered/radiused areas
>the perimeter of electrodes. Guess National & Millen had the righ idea
>they made all those old neutralizing caps with nicely machined or
Rounded and radiused goes back even before I was born, that was always
understood by HV and RF types. Collins understood that in their filters
and so did early Johnsons. The rest just stamped them out and cleaned up
The silicone idea is interesting. I wonder if anyone has taken that to a
pratical application at ham power levels and/or into repeatable
73 Carl KM1H
>From: firstname.lastname@example.org [SMTP:email@example.com]
>Sent: Wednesday, February 17, 1999 9:26 AM
>Subject: Re: [AMPS] isolation question
>On Tue, 16 Feb 1999 12:10:15 -0700 "Richard W. Ehrhorn"
>>In real life the actual rf breakdown voltage of a capacitor commonly
>>determined by unintended/unrecognized/unknown regions of
>>voltage GRADIENT - often many times higher volts per inch or mm than
>>simple calculation of (Etotal)/(dielectric thickness) would
>>Such regions can and often do occur (a) WITHIN the dielectric, due
>>inhomogeneities such as air bubbles or other contaminants, and (b)
>>dielectric surface, typically due to sharp (small radius) edges of
>>electrodes OR air gaps between the dielectric and the electrodes.
>>latter is almost inevitable unless electrode edges are of large
>>very smoothly polished, and the electrode-to-dielectric interface is
>>with a material soft or fluid enough to eliminate air pockets and of
>>dielectric constant roughly comparable to or greater than that of
>>This problem of dielectric inhomogenieties which create locally
>>voltage gradients, which in turn lead to corona discharge, which
>>holes in and eventually breaks down the insulation, is at least one
>>why HV caps are often potted and HV transformers are either vacuum
>>impregnated and potted or filled with oil. (Of course cooling also
>>a consideration for filling transformers and even caps with oil.) I
>>that relatively soft dielectrics like teflon and nylon generally are
>>susceptible to erosion by corona than are harder ones like mica,
>>high quality ceramic.
>>Despite design precautions, the end result is that rf dielectrics
>>must be many times thicker than their "dielectric strength" spec
>>indicate. It's also why the insulation in critical power
>>e.g., utility distribution transformers and ALPHA power
>>is applied on the basis of ~50V/mil rather than the several hundred
>>1000 volts/mil that the insulation's ratings suggest.
>>73, Dick W0ID
>Nice and concise Dick, thanks for that info.
>I have, and sure others also, blown the Teflon sheet as used in many
>type low pass filters. The failure points always seem to be around
>edges of the aluminum or brass disc; corona effect as you have
>Repair attempts have not been successful and it has been about 10
>since the last try.
>Do you have any suggestions? Would making the dielectric from 2 or
>thin sheets with a film of silicone be an improvement?
>BTW, I have only had this problem with HF and 6M LP and BP filters;
>in VHF/UHF amps when used as bypass/feedthru caps at voltages up to
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