LF: Measuring loop inductance

["Dick Rollema" <d.w.rollema@freeler.nl>]

To All from PA0SE

Jim, M0BMU wrote;


> One needs to be slightly
> cautious about this type of measurement - the inductance meter measures
> at a frequency somewhere in the LF range which depends on the inductance
> being measured - in general, this frequency will not be the same as the
> intended operating frequency of the loop. The inductance of the loop is
> fairly frequency dependent due to distributed capacitance, skin and eddy
> current effects, so the value at the measuring frequency will not be
> exactly the same as at the operating frequency. This is likely to give
> only very small errors for moderately sized, single turn LF loops, but
> could be serious for multi-turn loops with high inductance and stray
> capacitance, very large loops, or loops for use at MF or HF. Measurement
> methods which resonate the inductor at the intended operating frequency
> would then be highly desirable, although more
> cumbersome.

I fully agree with Jim.
If I had to measure the inductance of a loop in the LF band I would
resonate it at 136 kHz by putting a variable capacitor in parallel, if
necessary with extra fixed capacitors added. Only small power is needed;
a signal generator or small transmitter will do. As detector for
resonance an oscilloscope or VTVM can be used. After resonance is found
the capacitor(s) can be measured with a capacitance/inductance meter. The
inductance of the loop is then found from: 
L = (159.154/f)squared/C, with f
in megahertz, C in picofarad and L in microhenry. 

The capacitance can be measured at a  frequency different from 136
kHz, as a capacitance/inductance meter may do because - at least on LF -
there are almost no disturbing stray effects in a capacitor so its
capacitance is for all practical purposes independent of
frequency.

The loop inductance so found is not the real inductance but the effective
one, including stray effects. But that is what you are interested
in.

If for some reason you want to know the real value of the inductance the
capacitance in parallel with the loop can be increased so much that
resonance occurs at a low frequency, something of the order of 10 kHz or
so will probably be low enough. The distributed capacitance of the loop -
the main disturbing factor - is then so much diluted by the very large
capacitance in parallel that it that it hardly effects the
measurement.

73, Dick, PA0SE