Len wrote:

In order to measure any inductance component, the only
requirement is to find the DIFFERENTIAL between a direct
short across the bridge/RLC-meter connections and the
device itself (in this case a resistor).

Neil Hecht's excellent little LC Meter II does this automatically
by the zeroing button that subtracts the shorting inductance
from the device measurement, done arithmetically in the internal
microcontroller's registers.

I completely agree that all impedance measuring devices should be
"zeroed" in this way. But the problem with the resistor wires that we're
discussing here is *additional* to that.

We want to know the inductance of the metal-film resistor body, with the
wires cut very short as they would be for any application where low
inductance is important. However, for convenience, my first measurements
used almost the full length of the resistor wires to connect to the N
socket of the VNA.

It turned out that the total measured inductance is comparable to what
you'd find from the wires alone, so the body inductance is very small
(which is entirely consistent with the physical construction).

The suggestion had been to determine the inductance of the resistor body
by first repeating that original measurement, then applying conductive
paint to short out the resistor body, and then measuring again. The body
inductance would then be the difference between those two measurements.

Unfortunately that would be a poorly designed experiment, because the
resistor wires were bent around into a floppy loop whose size and shape
- and therefore inductance - is not very well controlled. The very small
inductance of the resistor body could easily become lost in variations
caused by small accidental movements of the wires. It is also an
experiment that cannot be repeated, because of the conductive paint.

If I'd found time at the weekend, I would have made up a little plate
like W4ZCB described. The N2PK VNA uses a three-step calibration with
open, short and 50R standard loads. For this jig, I'd have had to start
with the open-circuit connector spill, followed by a solder-blob short,
and finally by the best solderable 50R load I could make (probably two
100R chip resistors in parallel). Then I'd have cut short the wires of
the test resistor, and soldered that in place on the plate for the
actual measurement.

But unfortunately the whole weekend timed ou


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek