Dear Gary, LF Group,
The technique I use is like this -
Select a low-loss capacitor (polystyrene, silver-mica, polypropylene, air
variable, etc) that will resonate the coil near the frequency of interest
and connect in series with the coil.
Connect this series L/C across the output of a sine-wave source with a known
output impedance (sig gen, function generator), Rs, usually 50R. Monitor the
output voltage (using scope, RX + millivoltmeter, SLM, etc.) and adjust the
frequency (or capacitance) for minimum output voltage (i.e. series
resonance)
Measure the output voltage at the minimum (Vmin), then disconnect L/C and
measure the open-circuit voltage at the generator output (Voc). The
equivalent series resistance of the inductor (RL) is then:
RL = Rs.Vmin/(Voc-Vmin)
RL is usually what you want to know, but you can find Q = (2.pi.f.L)/RL
where f is the frequency.
You can also calculate L if you know what C and f is:
L = (1/(2.pi.f.sqrt(C)))^2
If your voltage measurement capability is not very accurate, or you are not
sure exactly what the sig gen output impedance is, you can use a
substitution method - note Vmin voltage L/C series resonant as before, then
replace L/C with different resistors until you find the value that gives the
same Vmin - RL is then the same as this resistor.
If you use a scope or wide-band voltmeter to measure Vmin, errors can be
caused for high values of Q (100 or more) due to harmonics in the output of
the signal source. This is fairly obvious on the scope screen. Using a
selective level meter, or the more cumbersome Rx + audio voltmeter,
eliminates this problem. The ideal tool for the job is a selective level
meter with tracking generator - I usually use a W&G SPM6/PS6. I have used
this technique successfully from audio to HF, with a wide range of
inductors. If testing a large loading coil, be careful to arrange things to
keep the coil well separated from the test gear, large metal objects, the
ground, yourself etc., otherwise Q can be substantially reduced by the
losses induced by the coil's field.
Cheers, Jim Moritz
73 de M0BMU
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