Date: Tue, 23 Apr 2013 09:52:15 -0700
From: Vic K2VCO <k2vco.vic@gmail.com>
To: Amps reflector <amps@contesting.com>
Subject: [Amps] Measuring tank coil inductance
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/
## The mfj 259B is useless. there is no way in hell the uh is gonna drop
that much when u measure it at the
freq of interest. You wind say a 40m coil, using a 259B, and you won’t even
be close to reality. You will have your
tank circuits so screwed up, they won’t even be close. The way to verify all
this is to measure the tune and load caps
as well, when it’s all in the test jig. Your pi net values will all come out
correct provided you can actually measure
the tube and load caps and the coil, and all stray c and stray l.
## I use a B+K 875B ( Mouser) This is a digital lcr meter. Measures down
to .1uh and also .1pf and high as 200 Henries
and as high as 20,000 uf. The resistance part of it will measure down to .001
ohm.
## You can also zero out the test lead stray uh and pf. When I want a say
8.4 uh coil, I want a 8.4 uh coil, and not something
that’s a mile out. Industry standard, comes with a 3 yr warranty. It’s my
#1 tool for building tank circuits coils, etc.
## I’d be dead in the water without it. BTW, the grid dip coils on the 259B
are also useless, even w8ji will tell you that, and he designed
it. I have yet to talk to anybody who can dip anything with it.
## measuring stray uh and stray pf is a piece of cake with the 875B. ditto
with tube C, vac caps, doorknobs, coax, and anything else
you can think of.
## The various charts and formulae will only get u so far....esp with tubing
coils., use the test gear for the final tweaking, taps, etc.
later... Jim VE7RF
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