PVC as a dielectric...Re: NEC-LIST: Hi-Q inductors

At 08:01 AM 2/15/2005, Chuck Counselman wrote:
>Dave Cuthbert wrote:
>>...I built a solenoid inductor wound on a PVC Pipe form. The measured
>>results are close to the K6STI results except for the exceptionally low
>>measured Q....
>
>
>I'm sorry not to have a quantitative result to report; but, IMO, this
>little experiment was quite damning for PVC insulation.
>
>73 de Chuck W1HIS

The tesla coiling folks have done a fair amount of experimenting with
losses of various and sundry materials used as coil winding forms (granted,
at frequencies in the 10s to 100s of kHz). Such experiments included
winding a coil and then measuring the (self) resonant frequency and Q
periodically over a longish time (many days) to determine if there were
correlations to environment, temperature, etc.

Here are some observations:
1) There is no such thing as "plain old PVC".. there's all sorts of grades
and forms and various degrees of recycled content. What appears to be a
nice white PVC pipe may only have a white PVC coating, and under that nice
coating might be grungy and grey. It's pipe, and the only specs are
mechanical, not dielectric properties. There can be radical (100:1)
differences in dielectric loss from seemingly identical pieces of
pipe. This was attributed to the use of recycled plastics in the
manufacturing process, where some of the recycling feedstock might have
metallic or conductive (carbon black) particles.

In the wire insulation context, since they are dying or coloring the
plastic, you have no idea what the original feedstock looked like, and
judging from some catalogs, nobody is claiming that the PVC insulation is
"virgin plastic". For PTFE, even if recycled, the feedstock is less likely
to include lossy materials (nobody is making UV resistant Teflon pipe, for
instance).

2) PVC is quite hygroscopic and porous. Depending on what other
contaminants there are in the PVC or on its surface, the bulk or surface
resistivity might change noticeably with humidity. ALmost all PVC coils
showed variations in HV breakdown and Q with humidity/temperature, unless
they were baked out and coated with something like glyptal.

3) Cardboard as a form is even more variable. Recycled paper, form releases
(in large Sonotube/QuikForm versions) and other impregnants might be to blame.

James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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