Re: Magnetic Field in a Flat Spiral

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

The variation in resistance just due to temperature might have a significant
effect on Q, even though the temperature variation isn't much. Let's see...
tempco of copper is something like 0.004 per degree (C).., I think.  A 10
degree change will result in a 4% change in DC resistance...  It will also
affect the skin depth, so the effect might be more than that....

It might also be 0.0004, in which case, we're looking at fractions of a
percent..

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, March 10, 2002 2:28 PM
Subject: Re: Magnetic Field in a Flat Spiral


> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
>
> Hi Paul,
>
> On 7 Mar 2002, at 17:06, Tesla list wrote:
>
> > Original poster: "Paul Nicholson by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> >
> > David Thomson wrote:
> > > what is the formula for calculating the Q of a coil by taking the
> > > two voltages on either side of the resonant frequency?
> >
> > Detect the coil's resonance with a distant pickup, or by measuring
> > the base current.  Turn down the output of your signal generator so
> > that it is below half of its maximum drive voltage. Tune the signal
> > generator for a maximum reading on your detector and mark this level.
> <snip>
>
> > A signal generator with a good fine tune is a prerequisite, as is a
> > frequency counter.  You need to make sure that the drive voltage can
> > be turned up by exactly a factor of two at the coil base. Use a sine
> > wave drive voltage.  Even with care the method might only be 5% or 10%
> > accurate due to the sensitivity to the small difference |f1-f2|.  Day
> > to day variation of the Q is to be expected, for reasons which remain
> > unexplored, I think.
>
> During a series of precision Q measurements I detected a variation
> between one day and the next of 6 - 7% in a moderately large
> resonator. The most notable difference between the two days was
> humidity. The air temperature on the day that the coil came in lower
> was also down somewhat - maybe 3 - 5C.
>
> Regards,
> Malcolm
>
>
>