Duncan Munro wrote:
On Tue, 30 Nov 2004 21:04:28 +0000, Ian White, G3SEK wrote:

Guessing a frequency and then doing the parallel - series
transformation on 22 ohms in parallel with 4.5uH produces results in the
right ballpark: R is still around 22 ohms but the *series* inductance is
100-200nH.

There is an additional complication in that there is another inductor in
the box itself of 680uH, LX (or should I say RX) is in series with that.
It's late now, but I will try and work out what's going on tomorrow night.

I will try to measure the resistors tomorrow.

If you get the opportunity, it would be much appreciated.


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.


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