Re: Dynamic Q
to: Jim, Malcolm
The only true way to find the loaded or "dynamic" Q factor of a spark
excited Tesla oscillator is to measure the log dec factor, ie, Q=pi/log dec
(decay factor). The rate of decay depends on the dynamic resistance of the
circuit and it is necessary to measure a spark excited system under actual
operating conditions. The static Q factor as measured with a signal
generator is useful as a reference but unreliable to obtain any true system
operating parameters. The biggest problem here is that the dynamic or
loaded Q factor of a secondary depends a great extent on the power level
that it is operating at, ie, different power levels will produce
dramatically different Q factors. As the series and parallel resistances
of the arc vary considerably they, in effect, become both parallel and
series resistances and in some cases both. This process is very dynamic
and is in constant flux. It does depend heavily on the driving power level
of the system. Using statistical sampling methods you can determine the
"average" or bell shaped curve with 3 sigma variation for most systems by
taking 30 storage scope samples and then using statistical methods to
determine the average value for up to 1,000 samples (statiscal process
control). This great variation in Reff causes the tuning and exact
operating parameters of TC's to be both art and science. If time permits
this winter I hope to write a long article for TCBA on Q and its influence
on the sec system, but it's been a busy summer and busy fall so not much
time for exchanging long e-mails on all the data. Hope this helps out
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Dynamic Q
> Date: Sunday, August 30, 1998 5:33 PM
> Original Poster: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
> Hi Jim,
> > Original Poster: "bmack" <bmack-at-frontiernet-dot-net>
> > I guess the calculation is the problem. As you pointed out in my
> > AC resistance post, it may be off by a factor of 2. For some reason
> > I have an adversion to unloaded Q tests. There are so many subtle
> > things that can introduce errors, stray capacitance, scope probes,
> > ground plane, height, orientation, etc. Considering all that, I feel
> > the results may be tainted.
> To only a small degree. In that series of measurements, I got the
> greatest isolation I could in a large classroom devoid of chairs.
> The resonators were sniffed from a minimum distance of six feet.
> Signal generator impedance at the base was 7 Ohms resistive. That
> caused a 10% error (low) in a H/D=1 coil which when taken into
> account caused that coil's Q to reach almost 600. The Q=300 resonator
> was far larger.
> > One method used was a test with the base grounded to a mesh
> > plane, which in turn was earthed. In all cases the bottom turn was
> > elevated at least one diameter above the plane.
> > Excitation was provided by VERY loose capacitive coupling from a
> > signal generator,consisting of #20 insulated wire wraped around the
> > loose coil wire (one or two "stretch turns"). This was dressed away
> > from the coil at a 90 degree angle to minimize other coupling.
> > If memeory serves, a 1khz square wave worked well.
> > Pickup was accomplished by clipping the probe around the magnet
> > wire (but not electrically connected) near the excitation point.
> > The coil would ring at it's natural frequency, and the periods could
> > be counted and the Q calculated.
> > The system dynamic Q was a more stable type measurement,
> > owing to the top load and direct measurement.
> > Here I put a series resistor between the generator ( 100 to 1000 ohms)
> > and the coil base lead. No, the resistor does not have any effect on
> I maintain that it should and measurements I have taken show that it
> does, for me at least. Compare that resistance with the ESR of the
> > In a few cases I tried both values for the series resistor and
> the > reflected
> > load calculation came out the same!
> > It's just a matter of measuring the drop across the resistor,
> > the current, then the refllected coil series resistance,Rs=E/I , where
> > E is the voltage measured at the base.
> > >From this, the top relected resistance can be calclated by using
> > a series to parallel conversion. Take this RL (top) and divide by
> > the charactoristic (surge) impedance and voila!, the dynamic Q.
> > Yeah I get around 100 for dynamic Q, but what bugs me is. this is so
> > lower than the coil's unloaded Q. I'm running high frequency for Tesla
> > stuff (900 khz +) so my Qu is high say around 800.
> > So what's the deal?
> > Jim McVey
> Don't know.