Negative Capacitance Readings on GW Instek LCR Meter with 5m MgO Insulated Cable

I am measuring the RLC properties of a 5 meter sample of mineral (MgO) insulated cable and I am getting negative capacitance readings.  I have performed the measurement on all possible frequencies of the measurement device. The measurement device is a GW Instek LCR Meter.  Can you explain the reasoning for the negative capacitance readings?

I don't know.  Most interesting though.  Googling "negative capacitance" brings up many articles on the subject.  Have you tried measuring the capacitance of plastic insulated cable to ensure that it is the insulating material that is causing this?  My guess from scanning the articles from Google is that there might be some magnetic property of MgO insulation that causes this.  That causes the voltage to go down when charged and up when discharged instead of the opposite with normal (positive) capacitance.
The Wikipedia article on mineral-insulated copper-clad cable mentions that magnesium oxide insulation has a high affinity for moisture.  I wonder what would happen if you were to drive any moisture out.  They say this is done by heating it.  Soak it in an oven (will it fit?) at 125 C(?) for an hour or two and try again?I'm just guessing here.  Very interesting.

Your LCR meter is likely operating at a frequency high enough to see transmission-line effects in your cable, which can easily give negative capacitance readings. Calculate the wavelength of your operating frequency in air, then multiply by 0.7 to include the dielectric effects of your cable's insulation, then compare to the length of your cable.

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