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[email protected] · December 7th 06, 06:51 PM posted to rec.radio.amateur.homebrew

wrote:
All,

I've been hearing how winding coils on discarded plastic pill bottles
is not suitable for RF tank circuits due to low Q, and that these would
not be safe for tube circuits anyway due to melting point.

I have access to kaolinite clay (white porcelain) and a kiln, and I was
wondering if I molded and fired this type of clay for coil forms, what
effect on a tank circuit Q this form would have.

Thanks in advance,

The Eternal Squire

There will be very little effect on Q of an inductor wound on a
1/16" to 1/8" thick wall of translucent plastic as used in pill
bottles...as compared to ceramic or epoxy-paper or other
plastics as used in commercial coil formers. A quick check
on any Q Meter will prove that. [I've done that out of curiosity
many many years ago and little change]

A visual clue is the old B&W "coil stock" of heavy wire held
in place by cemented-on strips of polystyrene (usually in four
strips per long coil stock, equidistant around the circumference.
Those were used in the 50s and 60s primarily in tube amplifier
circuits under about 200 W.

Using ceramic formers for anything under 400 W (give or take)
power level is overkill in insulation. Back in the 30s and 40s
use of ceramic was easy in the industry (plastics industry hadn't
developed far yet for electronics applications)...so already-in-
place ceramics work was an easy way to go for manufacturers
of "radio parts." That led many to believe that ceramic forms
are "necessary."

The BEST way to go is to sit down and do some grunge work of
calculation of RF currents and voltages in a power amplifier L-C
network...to see what ACTUAL loss will do insofar as heat
dissipation. Q - as measured - will be the same a low or high
power levels so the measurements at low levels will still apply to
high power applications.

If heat dissipation is a real concern, then that can be simulated
with simple AC through a resistance wound around the former in
question. Any resistance wire will do, such as the elements out
of an old toaster, hair dryer, etc. Set the low frequency power
dissipation the same as what was calculated for RF and watch
the results.

By the way, some pill bottles are still molded from polystyrenes
with some high end medications stuffed in (of all things)
polycarbonate (expensive) containers. The much-storied "Q
Dope" (supposedly so great to use on small coils, which it isn't
in longevity) is polystyrene with lots of solvent. Q Dope will pull
away from most surfaces ("lifts") after several months due to
adsorbed moisture in the air, losing some of its good properties.
Spar or boat varnish (from petroleum products) is better and
longer-lasting, doesn't smell as good as Q Dope but can
also be used on floors and furniture. I use McCloskey "Gym
Seal" myself...rather expensive in do-it-yourself stores, though.

Just having some kind of ceramic molded is never a panacea.
Without some good RF dielectric loss measurement gear you
won't be able to know if the ceramic mix has strange stuff in it
that could result in it being a lossy dielectric. Some ceramic
coffee mugs can heat up by themselves due to trace-whatevers
fired in with the ceramic. That indicates a dielectric loss at 2.4
GHz...but you won't know what the effect is at lower RF without
measuring it.

I would say you have a needless worry on former material. If you
want to fire your own ceramics, go for it! :-)