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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