Re: Measured Q (was Homemade chokes)

["Ed Phillips" <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

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?
> 
> Thanks,
> 
> Mark B.

	Not impossible by any means, but you are using the DC resistance
instead of the AC resistance, which is probably several times higher.  I
have made a number of small coils here on 3-1/8" diameter by 15" long
forms.  Using #28 wire the SRF is of the order of 300 kHz and the Q
typically measures around 300 to 350.  I have measured this both by
measuring the 3 dB bandwith of the resonance (making sure to account for
the generator source resistance) and measuring the effective series
resistance at resonance.  Both measurements agree within a few percent,
which is mighty good for Q measurements. 

Ed