More on Measuring Q

[Andre Kesteloot]

Dick Rollema wrote:

> To All from PA0SE
>
> James Moritz described in  his e-mail of 21 November 2001 at 13.38 hrs. a
> method of measuring Q of a coil that was new to me. Jim wrote:
>
> > What you really want to know is the equivalent series resistance of the
> > coil. I usually do this as follows: Connect generator to meter and measure
> > ampliude V1. Then connect a series resonant circuit using the coil to be
> > tested in series with a suitable resonating capacitor across the generator
> > terminals, and tune for a null in meter reading, ie. series resonance, and
> > measure voltage V2. At resonance, the reactance of L and C cancel, and the
> > remaining Rseries forms a potential divider with the paralleled source
> > resistance of the generator (Rs) and load resistance (RL) of the meter. If
> > you know what Rs and RL are, you can calculate Rseries:
> >
> > Rseries = (RsRL/[Rs+RL])*1/([V1/V2]-1)
>
> I used this method and two other ones to measure the Q of my loading coil.
> The coil can be seen on page 62  and on the cover of the LF Handbook (the
> caption of Fig. 4.17 at page 62 describes the coil on the cover).
>
> With a 47nF 2% capacitor in parallel the coil resonates at 11.56kHz. From
> this follows an inductance of 4033 microhenry (the large value of the
> capacitor eliminates the influence of the unknown self-capacitance of the
> coil). At 137kHz the inductive reactance of the coil is 3470 ohms.
>
> Using Jim's method I found a resistance of 9.17 ohms for the coil. This
> yields a Q of 3470/9.11 = 378.
>
> I also measured the resistance at 137kHz of the series resonant circuit,
> consisting of coil and vacuum capacitor,  using an admittance bridge.  I
> found the resistance to be 9.11 ohms. From this follows a Q of 381.
>
> I finally made a parallel resonant circuit at 137kHz, again using the vacuum
> capacitor. The bandwidth between the -3dB frequencies was 340Hz. This yields
> a Q of 137000/340 = 403. To avoid damping of the circuit by the tracking
> generator and selective voltmeter I excited the coil with a single turn
> loop, located 36cm below the bottom end of the coil. I connected a 1:10
> oscilloscope probe to the voltmeter and clipped it on the insulation of the
> top end of the coil.
>
> The Q values found of 378, 381 and 403 are within 4% of the average value of
> 387. This result is not too bad I think.
>
> 73, Dick, PA0SE