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

wrote in message
ups.com...
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.

What frequencies are your coils going to be operating at? I'd be really
surprised if pill bottles had any significant loss at HF.

You must be talking pretty high power designs if you're able to melt plastic!

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

Although the frequency is probably not the same as what you're going to use
the coils for, one way to test the loss of an insulator material is to put
some in the microwave oven (along with a mug of water to provide a load for
the magnetron). If the insulator material gets hot, then it is lossy. Of
course you would have to fire the clay before testing it, otherwise the
water content would make it get hot anyway.

If the pill bottles are polypropylene or a similar non-polar plastic then
they are probably not very lossy, but of course if the current through the
wire makes the wire hot by normal I^2*R losses, then the hot wire would
melt the plastic. This would only be a concern in a power amplifier or
tube circuits as you mentioned.

Chris

"Chris Jones" wrote in message
...
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

Although the frequency is probably not the same as what
you're going to use
the coils for, one way to test the loss of an insulator
material is to put
some in the microwave oven (along with a mug of water to
provide a load for
the magnetron). If the insulator material gets hot, then
it is lossy. Of
course you would have to fire the clay before testing it,
otherwise the
water content would make it get hot anyway.

If the pill bottles are polypropylene or a similar
non-polar plastic then
they are probably not very lossy, but of course if the
current through the
wire makes the wire hot by normal I^2*R losses, then the
hot wire would
melt the plastic. This would only be a concern in a power
amplifier or
tube circuits as you mentioned.

Chris

FWIW, I looked at a number of old pill bottles out of
curiousity, they are all marked PP No.5 which is
Polypropylene. Checking in a microwave oven sounds like a
good idea.


--
---
Richard Knoppow
Los Angeles, CA, USA

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! :-)