I saw the subject line, and was ready to warn against trusting a
return loss bridge at high reflection coefficients (assuming you'd
just hooked the L directly across the RL bridge unknown terminals),
but of course the method of putting an R and C in series gets around
that problem. If you have a swept system, you don't have to tweak the
cap, just look at the freq where the return loss is minimum...or where
the phase changes the fastest, or is zero. Then perhaps you don't
even need the series R (see next paragraph...).

In the "Measurements and Analysis" chapter of Reference Data for
Engineers, there are several bridge circuits suitable for measuring
inductance. One is a "resonance bridge" which really is doing just
what Richard wrote about. One arm of the bridge is a series RLC. But
in that case, the R is the equivalent series resistance of the LC, and
is balanced by the other bridge arms at resonance, just as a simple
Wheatstone bridge. Note that if your return loss bridge has been
calibrated to be accurate at small resistances, you can use this
without the series R in your LC to get a very quick estimate of Q, but
you can also do that by sweeping the frequency. -- See also circuits
for HP and Boonton (and GR?) Q meters.

Cheers,
Tom

"Richard Hosking" wrote in message .au...
My Friend Rod VK6KRG uses a snazzy method of calculating inductance using a
return loss bridge
I have tried it to measure a 500nH inductance and the result agrees closely
with the calculated result for an inductor of that physical size and number
of turns.

Basically he uses the unknown inductor, in series with a known capacitor, in
series with a 50 ohm load across the "unknown" arm of the bridge. When
XC=XL, then the two reactances cancel (assuming they approximate to an ideal
reactance) and load presented to the bridge by the RLC circuit = 50 ohms.
Thus you tune the system to best return loss which is a null and calculate
XL at this frequency You can even do this at the end of a section of 50 ohm
coax, assuming the loss isnt too great

No doubt all you RF experts out there have known about this for a long
time - has it been published as an idea before?

Richard