Here are some real numbers I just measured on a network analyzer
calibrated at the plane of the core being measured.
Core = T94-2 powdered iron u=10, 22 turns #18 occupying about 80% of core
Msmt. freq. L(measured)
50KHz 3.06uH
100KHz 3.86uH
200KHz 4.53uH
500KHz 4.51uH
1MHz 4.50uH
5MHz 4.50uH
10MHz 4.58uH
20MHz 4.89uH
30MHz 5.52uH
40MHz 6.80uH
68.1MHz parallel resonant freq.
As can be seen frequencies above 10MHz show the beginning effects of the
parallel resonance and low measurement frequencies underestimate the
inductance by a significant amount. Also mounting the inductor in a
metal box will reduce the inductance. I suspect if the measurement freq.
was 1KHz the values would be even lower. I always measure near the
frequency I intend to use the inductor at and if possible with the
inductor mounted in the enclosure. Then calibrating to the inductor
plane gives the "real" inductance and filters and matching networks come
out as predicted.
73,
Larry, W0QE
Vic K2VCO wrote:
> On 3/30/2010 4:47 AM, Jack/W6NF wrote:
>
>> I have used an HP LCR meter at work but never paid any attention to the
>> frequency at which the measurement is made. I wonder what discrepancies in
>> measurement values of L and C could be encountered if one measured at
>> various RF frequencies, and why they might occur. I have seen such
>> discrepancies, using a popular antenna analyzer, even though I was careful
>> not to introduce sources of error, like lead lengths.
>>
>
> If the coil has a ferrite or iron core of any kind, the inductance varies a
> lot with
> frequency. Inductors with cores must be measured near the operating
> frequency. And in
> general, as you raise the frequency the various stray capacitances and
> inductances in the
> device under test become more significant.
>
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