hello,

i'm a student in Belgium and need to measure Motor-cable parameters with a VNA, first of all, i want to measure the LC parameters of

a "cable" wich exists of 2 blank conductors with 5mm space in between and surrounded by air. I did a 1-port callibration, and measured Z11 two times, first time with de cable shortcircuited(SC) at the end , and second time with the cable open circuit (OC) at the end.

I've read somewhere that the imaginairy part of the SC measurement represents L, and the imaginairy part of the OC measurement representes C. I did these measurements with an LCR - meter also, and there the parameters are correct (within 10pct fault) if i compare them with a theoretical calculation. but with the VNA it isn't correct, the L measurement has 20pct fault, and the C-measurement up to 100pct fault...

what am i doing wrong?? and how should i do it? is it possible to measure very small impedances(SC), and very large impedances (OC) with the VNA?

All help would be very welcome

i'm a student in Belgium and need to measure Motor-cable parameters with a VNA, first of all, i want to measure the LC parameters of

a "cable" wich exists of 2 blank conductors with 5mm space in between and surrounded by air. I did a 1-port callibration, and measured Z11 two times, first time with de cable shortcircuited(SC) at the end , and second time with the cable open circuit (OC) at the end.

I've read somewhere that the imaginairy part of the SC measurement represents L, and the imaginairy part of the OC measurement representes C. I did these measurements with an LCR - meter also, and there the parameters are correct (within 10pct fault) if i compare them with a theoretical calculation. but with the VNA it isn't correct, the L measurement has 20pct fault, and the C-measurement up to 100pct fault...

what am i doing wrong?? and how should i do it? is it possible to measure very small impedances(SC), and very large impedances (OC) with the VNA?

All help would be very welcome

For an open or short circuited cable, the reflection coeffient is 1. Impedance measurements for reflections greater than about 0.8 can have a lot of error.

What is your frequency of interest? A "motor" cable sounds like it will be used at very low freuqencies. As an alternative, and if your frequency is not too low, you could terminate the cable in Z0, measure the impedance by sweeping through 1/4 wavelength section, and looking at the S11 or impedance at the frequency where it is exactly 1/4 wavelength. At this frequency, Z0 impedance at the end of the line will look like a different impedance: Z(1/4) = ZL^2/Z0 So if your 50 ohm Z0 load measures 40 ohms at the 1/4 point, the line is 44.7 ohms.

Now, you know the impedance of a lossless line is equal to sqrt(L/C); YOu can also measure the delay of the line with a short at the end, so that you know the electrical delay and can compute the velocity factor from the length and the delay. The velocity factor is equal to the 1/sqrt(LC); so 2 equations, 2 unknowns, the rest is left to an excercise for the student.