Ive had a 4815A for about 15 years courtesy of a company auction.
I find it cumbersome for amp work but ideal for chokes and also phased
verticals LC networks. It mostly sits on a basement steel shelf these days.
Carl
KM1H
----- Original Message -----
From: "John Lyles" <jtml@losalamos.com>
To: <amps@contesting.com>
Sent: Wednesday, April 24, 2013 8:27 PM
Subject: [Amps] Measuring tank coil inductance
As others have expounded on their preferred ways to do this, I will
supplement what was said (no disagreement here) with one more technique
that I have used for 20 years, and built some high power RF amplifiers
with in the HF range, for work. A vector Z meter, one of the old HP models
like a 4815A or the better 4193A, will not lie in the nominal range of
values. It will show not only the magnitude but the phase angle of a
reactance, be it inductive or capacitive, at any freq from 500 KHz to 110
MHz. This gets you back to the problem observed, where stray capacitance
is inherent in a coil, and it resonates to give a somewhat or drastically
different value of Z at the terminals. Vector Z meter will easily show
this. It is perfect for checking RF chokes.
I also use various meters such as Boonton, Sencore, and little digital LC
meters, that measure at low fequency like 1 KHz or thereabouts. These give
you the component value that is expected for the geometry, such as plate
spacing, area and dielectric for a capacitor, and turns, wire guage,
diameter, and turns spacing for inductors. It won't give you the strays
effect as seen at HF. But if you stick to the measured values, and allow a
little tuneability in the final network, this should get you close. Do the
mock up suggested, with a resistor of Rp value across the tube terminal.
The vector Z meter, or other HF bridges and instruments, will show the
actual reactance, and how it varies as f goes up. Not to scare you, but if
you use a coil way beyond where it flips phase angle, you might have some
surprises. The only negative with the old HP is the active probe having a
very fine tip that breaks easily for large component connections. Get a
bunch of them, they screw in with a fine thread. I have converted to a
screw so that it touches the coil but doesn't clip to it, to prevent
tearing up the probe on a big coil. Or use the Type N to probe adapter.
Then you can extend the type N side with a connector and some clip leads,
etc. I have several homemade clips for ours, that are a type N femail
connector, mounted to a large piece of copper flashing for low ground
inductance. This flashing has a big alligator clip on one end (or two in
parallel) and the center pin has another. I bend the coil as best possible
to touch both ends. For a long coil, say 6 or mor
e inches
long, I have another fixture that is an aluminum plate with a vertical
piece of copper sheet folded for strength. Along this piece are several
holes for type N connector. I mount the N wherever it is needed there,
with the single alligator to reach the one end of a coil. The other end of
coil goes to a screw on the base plate, so it is similar to real world
mounting.
These meters can be found for a hundred dollars for a 4815A, but it has to
have a working probe, the hard part. I am still looking to find a 4193A
that is reasonable, as its the cat's meow.
73
John
K5PRO
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