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. Paul k5wms Quoting Wes : Hi all, 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 unused. 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 junk parts. 73, Wes w7zoi Paul Daulton K5WMS beacon WMS 185.302 khz qrss30/slow 24/7 Jacksonville,Ar 72076 em34wu [Non-text portions of this message have been removed]

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.

Paul k5wms

Quoting Wes :

Hi all,

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
unused.

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
junk parts.

73, Wes
w7zoi

Paul Daulton K5WMS
beacon WMS 185.302 khz qrss30/slow 24/7
Jacksonville,Ar 72076
em34wu

[Non-text portions of this message have been removed]