Re: Measured Q

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Bart,

On 14 Aug 00, at 7:53, Tesla list wrote:

> Original poster: "Barton B. Anderson" <tesla123-at-pacbell-dot-net> 
> 
> Hi Malcolm, 
> Question below: 
> 
> Tesla list wrote: 
> >
> > Original poster: "Malcolm Watts" <M.J.Watts-at-massey.ac.nz> 
> >
> > Hi Mark, 
> >
> > On 12 Aug 00, at 19:26, Tesla list wrote: 
> >
> > > Original poster: "Mark Broker" <broker-at-uwplatt.edu> 
> > > 
> > > > You can calculate         Q = XL / R 
> > > > 
> > > > Note:       XL = 2*pi*f*L 
> > > > 
> > > > Where L is the inductance of the coil in Henries. 
> > > 
> > > Hello, list 
> > > 
> > > I finally decided to measure the Q of my coil.  I have a measured DC 
> > resistance 
> > > of 26.7 ohms.  According to E-T5, my coil's operating frequency is about 
> > > 125kHz.  The inductance is calculated to be 33mH by WinTesla.
Putting this
> >
> > > into 
> > > the equations gives me a Q of 970! 
> > > 
> > > This number seems about 5 times too high, and my primary tuning isn't 
> > terribly 
> > > finicky.  Is this high of a Q possible? 
> >
> > Not in your coil unfortunately. You cannot use DC resistance 
> > in those calcs since frequency at DC = 0Hz.  In practice you 
> > have to use what is known as ESR (effective series resistance) 
> > which lumps all losses (skin, radiation etc.) at the frequency 
> > of interest into a single resistance. You can quantify the ESR 
> > of your coil by doing an accurate Q measurement and deriving 
> > ESR = wL/Q. 
> >  
> 
> 
> 
> 
> 
> Doesn't w*L = XL?

Yes.

> Also, the skin depth at the operating frequency will cause
> the ohmic value of the winding to increase. I'm curious if the method I used
> below will hold water. 

 
> The skin depth of my coil is .0104 inches using the equation 
> Sd(inches) = (66 / sqrt(Fo)) / 25.4 
> 
> If I calc the resistive value of .0104 inch wire diameter using the same
length
> of wire the coil is wound at, I end up with a large resistance of 335 ohms. 
> 
> If I then take XL/R = 34,543 / 335 = Q of 103 
> 
> Obviously eddy currents and hysterisis are not accounted for and the only
> proper way to find Q is measurement, but this approach seems to get close (or
> does it?). 

Have you measured the Q of your coil? I think it would be 
somewhat lower if the coil is closewound because although skin 
effect would be pretty well absent there would still be some 
proximity effect. Please let me know. I'm curious about this 
case.

Regards,
Malcolm