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Can a stranded wire exhibit a capacitive reactance?

I measured the real and imaginary part of a piece stranded wire using an old LRC meter( HP 4284a). Applied an open loop and a closed loop compensation for the parasitic impedance of the lead wires. Measured reactance is negative. This left me puzzled, is this phisically possible?

I know that a real conductor will display an inductive behaviour because of self inductance, with positive reactance, increasing with frequency. Could a stranded geometry cause a negative reactance to be measured?

Or is this more likely a wrong calibration issue?

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  • \$\begingroup\$ "Or is this more likely a wrong calibration issue?" Have you just let slip the real question? Calibration of what? Please edit your question to give it some context, measurements, name, model and links to datasheets of your test equipment, etc. \$\endgroup\$ – Transistor Nov 26 '17 at 23:16
  • \$\begingroup\$ Hi, thanks for your answer, I added some extra details: I measured the real and imaginary part of a piece stranded wire using an old LRC meter( HP 4284a). Applied an open loop and a closed loop compensatio for the parasitic impedance of the lead wires. Measured reactance is negative. This left me puzzled, is this phisically possible? \$\endgroup\$ – Alyons Nov 26 '17 at 23:22
  • \$\begingroup\$ L don't know but someone will. Put all the relevant information in your question so it's all in one place rather than sprinkled through the comments. By the way, these are comments rather than answers. \$\endgroup\$ – Transistor Nov 26 '17 at 23:57
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    \$\begingroup\$ It also depends on the physical setup. Also, since it is " an old LRC meter( HP 4284a). " it's calibration date is likely way past expired. \$\endgroup\$ – Trevor_G Nov 27 '17 at 0:10
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All wires have a distributed inductance and capacitance and a resonant frequency which becomes a short circuit at 1/4 wave and beyond this inverts the wire reactance and impedance rises to the free space impedance at 1/2 wave length and then repeats.

This is the characteristic of a whip antenna.

So it depends on the length of wire and what f was used in the test.

The wire is inductive based on ratio of length to diameter and capacitive based area/gap ratio with the ground signal. So a single wire is fairly low pF /m compared to loose twisted pair which is ~ 50pF/m.

Conclusion:

  • measurement error or at least ill-defined measurement conditions.

Although proximity of wire to hand or any dielectric that is grounded by stray a capacitance will affect pF/m.

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  • \$\begingroup\$ The wire i measured was about 6 cm long and the range of frequency i am interested in is between 0 and 1MHz, so I doubt there is any influence on the impedence of the resonant frequency of the wire.i saw small values of capacitive reactance, progressively increasing with f(0, to -0.004 ohm @1MHz) \$\endgroup\$ – Alyons Nov 27 '17 at 5:47
  • \$\begingroup\$ 60mm wire is about 60 nH and capacitance depends on proximity. So I expect calibration error . Did You calibrated with open,short and reference R \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 27 '17 at 6:53
  • \$\begingroup\$ You did not say if you measured 1 end of wire or across both ends \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 27 '17 at 7:47
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No, a piece of wire can't exhibit a capacitive reactance, a guess. I think there is a problem with meter/connection. What results are measured while closed-loop compensation? What output meter gives at 'short connection' after 'closed loop compenstaion'? What output meter gives at small coil with known inductance? What output meter gives at the stranded wire?

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This is a 'small difference between two big numbers is meaningless' issue.

Your meter's open and short calibration will be slightly contaminated by noise, as will your measurement. Your corrected reading is therefore dominated by measurement noise, and is approximately zero, plus or minus some noise.

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  • \$\begingroup\$ But this measurement noise guves me a reactance that increases with frequency, from 0 to 0.004 ohm. Shouldn't the measurement error being randomly distributed around 0? \$\endgroup\$ – Alyons Nov 27 '17 at 5:56

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