[Lowfer] Interesting finding

[Ed Phillips]

Bill Ashlock wrote:
> 
> Ed,
> 
> >A Q of 8000 with a LOSSLESS loop would still mean a dissipation factor
> >of 0.000125 for the capacitor, or less than the dissipation factor I can
> >find listed for any mica type.
> 
> Right. I may be simply trying to violate the properties of the material.
> 
> >A good EXTENDED FOIL polystyrene
> >capacitor (or group in parallel to reduce the effects of lead
> >resistance) is probably your best bet but still can't make it.
> 
> I could only find one polystyrene, last night, in the range I need, a 900pf.
> The Q was better than the group of 4 paralleled micas but not as good as the
> group of 3 tiny COG ceramics. I have a whole box of polystyrenes here at
> work but they, so far, elude me.
> 
> >In principle an air-dielectric capacitor might, but it would be enormous.
> 
> Is glass-epoxie any good? (as in double sided pc board stock)
> 
> Thanks!
> 
> Bill

	I'm surprised the ceramics are that good, but don't recognize the
type.  What size capacitor are you looking for?  Over the years I've
picked up a lot of polystyrene capacitors and still see them advertised
on occasion.  I use them both for RF and audio filter purposes.  I used
a bunch of Mallory 0.01 ufd 400 volt capacitors to tune the tank of my
first lowfer transmitter back in 1976.  At that time All Electronics had
an almost unlimited supply of those for a dime each and I bought a
bunch, of which few are left.

	As for air variables, they are available up to pretty high values (if
you call 1200 uufd high) from several surplus sources.  I'm not sure
their Q is necessarily the best that could be obtained, however.  I have
one which I know isn't the best.  It's about 0.006 ufd max and is made
from a whole gang of four-section 365 uufd capacitors stuck in a tank of
transformer oil with their shafts geared together.  It's big (about a
foot cube), heavy, and pretty much useless.  Someone gave it to me and I
wish I hadn't brought it home as I'm afraid it will spring a leak, it's
pretty much useless, but it's "too good to throw away".  I'm sure none
of the rest of you have that latter problem and can't understand the
mentality....... 

Glass-epoxy is very high loss and NG for sure!  I tried making some
capacitors from 10" x 12"  sheets of PC board material and the Q was
only of the order of 20 at 200 kHz.  I was going to try them for primary
capacitors in a Tesla coil but they were way too lossy.

	By the way, I understand the problem of the box of stuff which has been
misplaced just when you need it!  Assume everyone has his set of lost
boxes which will turn up unexpectedly when no longer needed.

Ed