Hello Dick,
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.
Your three Q measurements indicate that your measurement techniques are
sound - unlike mine!
It occurs to me that I could use my HP vector impedance meter to measure Q
using the series resonant method. Unfortunately it only goes down to 500kHz
so cannot measure Q at the operating frequency. If I did the measurement at
a higher frequency would the reactance of the series resonating capacitor be
significant and give a higher value of perceived resistance?
I could also use my 3M bridge (page 86 of LF book), which can measure R at
the operating frequency (136kHz and 73kHz). The resonance curve could then
be plotted using the S/W shown on Fig 6.12.
More experimenting to be done when the weather gets dryer.
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
| 