Paul,
The results were a bit of an "eye opener" to me - they've certainly made
me sceptical about tuner loss calculations based on "constant Q" inductors.
I'm pretty confident of their accuracy. They were made with an AIM4170
which I typically find has about 0.2pF of residual output capacitance
that doesn't get taken out in the measurement-plane calibration process.
That amount of capacitance would have no significant effect on these Q
results.
For reference, the inductor I measured was a Multronics 229-202-1 with a
ceramic former:
http://www.cardwellcondenser.com/PAGES/md04g.html
73,
Steve G3TXQ
On 29/11/2010 19:57, Paul Christensen wrote:
> Steve,
>
> Sure makes one think about where and how the loss of Q is occurring.
> Unused turns can couple to the active portion of the coil, reducing Q.
> Another factor that comes to mind is the shorting rod that impresses a wheel
> against a coil or in the case of an edgewound variable, the shorting bar to
> the wiper contacts. Coils wound on solid dielectric material is also a
> factor unless the coil is air-wound.
>
> In my homebrew motorized tuner, a pair of Collins silver tape inductors are
> used on ceramic forms which supposedly keeps Q reasonably consistent as a
> function of F and L. Unused tape spools up on a conductive cylinder. There
> are no moving contacts. I've not tested one of these coils but it would be
> worth the effort to see if the claim is true or not.
>
> Link-coupled tuners with plug-in coils designed for single-band operation
> generally do not suffer from Q loss. Link coils made from B&W Airdux or
> free standing coils made from stiff copper tubing are capable of keeping Q
> very high.
>
> Paul, W9AC
>
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