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
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