# Re: Coil form material - Q-killer?

```Original poster: "Gav D" <gdingley@xxxxxxxxx>

Hi Ed, Jim, Mike
thanks for that information, I will alter my PSpice simulation
accordingly. Were either of you able to measure the Q directly, i.e.
before breakout occurs during operation? I have attempted this by
recording the waveform digitally and performing an FFT, but I think in
the end I was just testing the effectiveness of the FFT! I should
resort to more analogue methods, and suggestions/ideas?

I retrieved a paper written by Wheeler on calculating skin-effect
resistance, a methodology rather than a formula, i.e. it can be
applied to any conductor geometry. All you actually do is take the
formula for the inductance (e.g. the much used Wheeler formula for
example), differentiate it by the direction in which the current will
most predominantly flow (radius r in the case of the Wheeler
inductance formula), divide by mu, and multiply by the skin-effect
constant for that material at the particular frequency of operation.
The results look OK, but I have not done any actual measurements to
verify it. I will try and scan the Wheeler paper and find my copy of
the Medhurst .zip file.

Gavin

On 1/9/07, Tesla list <tesla@xxxxxxxxxx> wrote:
```
```Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 03:02 PM 1/8/2007, Tesla list wrote:
>Original poster: "Gav D" <gdingley@xxxxxxxxx>
>
>Hi all,
>I have been reading the Medhurst paper from which the much used coil
>self-capacity formula originates and noticed a formula for dielectric
>loss. Using this formula and calculating skin-effects, I have found
>that the PVC pipe often used for winding a secondary contributes more
>to reducing Q-factor than the conductor itself; I am correct?

PVC isn't all that lossy at a few hundred kHz.  There is a loading
effect from the dielectric, but that just increases the parasitic C,
lowering the self resonant frequency.  I would imagine that resistive
losses still dominate.

>
>Regards,
>
>Gavin
>
>
>

```
```

```