[Amps] Measuring tank coil inductance

[Carl]

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 at losalamos.com>
To: <amps at 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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