At 06:07 AM 7/7/2005, Jim Lux wrote:
>At 11:56 PM 7/6/2005, Tom Rauch wrote:
> > > One way is if you have an air core inductor that you can
> >measure the loss
> > > of (say you've got it resonated with a capacitor at 14
> >MHz). Shove the
> > > suspect hunk of plastic in the middle and see if the Q of
> >the coil
> > > drops. If not, you're good to go.
> >
> >Space winding an inductor reduces turn-to-turn capacitance
> >and increases inductor Q. Fill the area between turns in
> >with a dielectric and Q decreases. That's because
> >capacitance and circulating currents increase. Knowing that,
> >how do we quantify how much Q loss is caused by an increase
> >in turn-to-turn capacitance as a dielectric is brought near
> >or against the turns and how much is lost through power
> >dissipation in the dielectric? How many people can measure
> >Q?
>
>I think one could get a "quick and dirty" feel using a typical antenna
>analyzer and measuring the SWR bandwidth of the coil and capacitor
>combination.
I can't find it here, but somewhere I have a copy of a Masters thesis where
the candidate actually used this technique to measure moisture content of
wood (or maybe soil samples, I can't recall) by measuring the RF loss. He
used a SWR bridge and a 40m transmitter, as I recall. Fairly good
analytical backup and validation of the technique. It's fairly elegant,
since all you really need to do is measure 3 frequencies: the resonance,
and two points on either side where the SWR is some value that's reasonably
measurable.
With calibration, it can be made quantitative, but you'd need to control
the sample size and position with the coil.
Jim.
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