EMRFD Message Archive 5611
Message Date From Subject 5611 2010-12-30 16:58:12 Paul Daulton Re: "Polyvaricon" capacitor Wes, interesting thread. I would like more info as to how you
measured the Q. I am familiar with Q measurement of inductiors but I
have never checked caps.
In a project this summer I tried to use polyvaricons as the main
tuning of a simple receiver. First problem I encountered was the lack
of conventricity of the shaft. Mine were provided wth a nylon bushing
and a metric screw to make a shaft for the tiny hub. Mounting this cap
to a plastic reduction drive I was unable to get the back lash out of
the combination. Another problem was drift, the poly varicon drifted
more than a variacap tuning in the same circuit.
In an application like an antenna tuner or a direct drive with a
knob the polyvaricon would probablly be ok. My BITX20A from Hendricks
uses the polyvaricon for the main tuning and a varicap/pot combination
for fine tuning, I have no complaints about this rig.
Quoting Wes :
I did an experiment that turned out to be both interesting and perhaps
useful for the folks in the greater QRP community. A popular capacitor
type is the so called polyvaricon variable. I'm not sure if this is a
proper term or just something that has evolved, but that is not the
point. The capacitor is a variable element that is physically small and
rugged. It is built much like a traditional air variable, but the plate
spacing is smaller. The space between plates is then filled with a
plastic. I have a batch of them that I purchased from DigiKey or Mouser
back when they carried such things. I think that's the proper heritage,
but don't hold me to it. The part in my junk stock was definitely
Anyway, I was curious about the quality of these parts. Such things are
popular as tuning elements in QRP transmatches. So a Q measurement was
done. I've been on a kick recently to measure the Q of capacitors.
Anyway, the Polyvaricon that I measured was especially bad. The result
was a Q of about 500 at 10 MHz when the capacitor was adjusted for C of
100 pF. Q was even worse at 350 or 400 when the capacitor was fully
meshed for C=270 pF. All of the details can be found in the
"experiments" section of my web site at w7zoi.net.
Just to be sure that I was not making some colossal mistake, and I've
been there before, I used the same VNA calibration and measured the Q
of a good air variable immediately after I had measured the
polyvaricon. The air variable was excellent at Qc=3100.
I will measure some additional versions of this capacitor type. If the
rest are as bad as this, I'll be rebuilding some gear to replace the
Paul Daulton K5WMS
beacon WMS 185.302 khz qrss30/slow 24/7
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