Ian, if it's not too much trouble, I'd be very interested in how the
measured inductance compares to that of the resistor body/leads only.
I'd think that could be done by coating a similar-size resistor with
conductive paint or foil and measuring with the same method. The
difference between this measurement and the one you made would then show
how much inductance is due to the spiral element, and would represent
the minimum possible inductance for that resistor body type and lead length.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:


Over a range from 50kHz to 50MHz, Duncan's two resistors measure about
22 ohms + 31nH, and 33 ohms + 23nH.

To avoid having to construct a special test jig, I measured each
resistor with about 30mm of bent wire leads, which would account for
about 20nH of those measured inductance values.

The very low inductance of the resistor body is completely consistent
with the physical construction. On closer inspection, the metal film is
an almost continuous tube, with a very narrow spiral gap of about 1.5
turns. The gap adjusts the resistance by slightly increasing the overall
electrical path length, but it adds very little inductance.

If you used very short leads instead of the longer lengths I had to use,
these resistors would have a low SWR up to at least 144MHz.

Thanks again to Duncan for supplying the resistors.