A reply follows, I apologize for the length. I have worked on this problem a
lot over the years and its a tricky one to solve at high power coincident with
high frequencies:
> From: Larry<larry@w7iuv.com>
> Subject: [Amps] More HV feed through capacitor
> To: amps@contesting.com
>
> FWIW here is an "update on my feed through capacitor problem:
>
> The amp in question is a GS-31B based on one of the YU1AW designs
shown on the ND2X web site. This design specifies a anode choke bypass
> capacitor constructed from FR4 PCB material...
I had similar problems with planar HV bypass capacitors made from laminated
material. Many years ago, I stripped an ancient Amana Radarange microwave oven.
It was one of the earliest, having a dial timer. The maggie had a little box
around it with both heater leads coming through little feedthru caps made from
machined aluminum disks and PET film, along with a Teflon insert to center the
bolts through holes. That stimulated me to design HV feedthrough capacitors for
broadcast FM amplifiers that way. I used Copper-clad Kapton polyimide film,
called Pyralux, made by Dupont. The capacitance was very good for VHF and the
self resonance was way up there. It was designed as a Pi filter, with the bolt
through the middle being the L. I didn't realize how strong an effect sharp
edges had with HV. The field enhancement (only 4100 VDC) slowly ate the edge of
the Kapton on the inside capacitor, from ionization there. After over a year in
the field, they began coming back from customers f
ailed.
You could see a ring around the metal edge where the Kapton was 'etched'.
Lesson learned - don't have sharp edges on the electrodes, even with wide
spacing of nearly an inch. HV will do damage there, over time. It is amazing
how far it can track over a surface, given some ionization from partial
discharge (corona).
Using FR4 at 430 MHz is asking for trouble too. Even though the capacitor is
supposed to be at a voltage node for the fundamental freq, based on the tank
circuit design, it might be an antinode for the even harmonics. Not only is
FR4/G10 a poor dielectric (loss tangent is marginal due to the epoxy resin) but
it is also capable of thermal runaway as it heats and the loss worsens. Also it
may absorb moisture. All bad things for a dielectric under HV stress.
As stated by another commenter, making it with Teflon-filled PCB material is
probably better. Pure Teflon is difficult as it has low dielectric constant so
you'd need a thinner piece or more area to get the same capacitance. Some of
the standard Rogers microwave PCB materials with K of 6-10 would be better.
Still, you run risk of flashing around the edges, if not right away, a little
while later.
You are moving in the right direction, in my opinion, by putting a small choke
in series, into a 'doghouse' as you called it. The choke should be
perpendicular to the transmission line circuit, with as few turns as possible,
perhaps just a few turns about a half-inch diameter, and inch long. Just enough
to give some inductive reactance in series, but not dangerous from too much
stray cap to cause it to resonate at the fundamental or second harmonic. If you
can, measure it with a network analyzer, antenna analyzer, or other UHF
instrument. At the cold end, put your capacitor to ground. Get rid of sharp
edged copper and use radiused plates or even a radiused cylinder clamped inside
another cylinder (see below). Where there is a radiused edge of metal, fill
that space with silicone RTV, or better yet, 2 part silastic potting like Dow
Sylgard 184 or GE/Momentum RTV12. These are messy, but very good materials with
high power RF - improvements over ancient Gylptol paint. By sealin
g the
edges, you raise the dielectric constant over air, and lower the RF voltage
across the tiny air gap where the metal rolls away from the film. [Look up
"triple point" and "dielectric flashover" on Google to see explanations.]
Use Kapton, PET (Mylar) or Teflon film, a few mils thick will do. Its better to
use a few layers of thinner material than one fatter sheet, although at amateur
radio voltages it might be OK. Kapton only comes up to 5 mils (0.005) thick
anyway. You'll want less than a nanofarad to make a decent bypass at UHF this
way.
I have been relearning this in the past 2 years, as I had to build a big HV
feedthru cap for my 200 MHz cavity amplifier at work. Nothing was commercially
available. I am passing 125 amps of plate current through it, at 28 kV DC. I
borrowed an idea from the bigger Broadcast Electronics FM transmitters that I
remember. I made an aluminum cylinder and then a matching aluminum slug that
fits into the center with a slight interference fit. By wrapping the slug with
Kapton, several layers, then heating the outer cylinder, I pressed the center
slug into place, and it sealed tight when it cooled off. Then I potted the
edges. I did the potting under vacuum to ensure that there are no air pockets
that will ionize and cause failure of the Kapton. It has been working for a
year now, fingers crossed.....
I hope some of these ideas will help or inspire you to break out of the box and
try something else. Forcing disk capacitors to work at UHF is risky. Good luck,
let us know what works out.
73
John
K5PRO
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