Re: Magnetic Field in a Flat Spiral

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

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