I've been trying to find a good method to measure the inductance of large coils made out
of tubing. For example, I have a coil made of 19 turns of 3/16" (about 4.8mm) tubing,
3.25" (8.3 cm) diameter and 5.5" (14 cm) long. The calculated inductance is about 8.3 uh.
I've tried the following methods to actually measure the inductance:
AADE LC meter IIB: This device is very useful for such jobs as determining the values of
small components, calibrating vacuum capacitors, measuring the inductance of RF chokes,
etc. But it consistently reads low when measuring coils where the distributed capacity is
large compared to the inductance. I did this experiment: I made a coil of 4 turns of 3/16"
tubing, 3" long. I measured the inductance (low compared to calculated value), and started
compressing the turns. The measured inductance increased (as it should) up to a point, but
then started DECREASING! My thought is that the effect of the increasing distributed
capacity overcame the increased inductance. I also measured the inductance of an
unmodified B&W 852 tank, which has a built-in switch and a lot of distributed capacity.
The measured values were much lower than the spec sheet indicated.
MFJ 259B antenna analyzer: measured inductance increased rapidly with frequency. Totally
worthless.
Measuring resonant frequency with GDO with parallel capacitor: seemed to read low compared
to formula, but I think it may be the best method.
Now I am wondering: in a practical tank circuit, if the pi-network calculation calls for,
say 8 uh, shouldn't you take into account the distributed capacity? In other words, the
inductance isn't important: what you REALLY want is a specific inductive reactance at the
operating frequency, which in the case of a large tubing coil would be composed of the sum
of the inductive reactance and the (negative) capacitive reactance of the distributed
capacity.
If this is correct then probably the GDO method, selecting a parallel capacitor which puts
the resonant frequency near to the operating frequency, would be best. The procedure would
be to use a variable capacitor, achieve resonance in the band in question, then measure
the capacitance (with the AADE) and compute the inductance.
I know that the final adjustment should be made to reduce the SWR measured from the output
with a resistor equal to the load impedance from plate to ground, but this can be achieved
with different values of Q, so you need the inductance to be close to the calculated value.
How do real amplifier designers do this?
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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