Re: Measured Q (was Homemade chokes)

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

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.

Regards,
Malcolm



> Thanks,
> 
> Mark B.
> 
> 
> 
>