# Characteristic Impedance of a cable calculation

Discussion in 'General Electronics Chat' started by Gibson486, Oct 8, 2015.

1. ### Gibson486 Thread Starter Member

Jul 20, 2012
199
12
I have this cable...

http://alphawire.com/en/Products/Cable/EcoGen/EcoCable/77109

However, when I try to calculate the characteristic impedance (sqrt of L/C), I do not get their impedance that the have on their specs.

Am I making the wrong assumption that I just add the ground and wire capacitance?

Thanks!

2. ### crutschow Expert

Mar 14, 2008
13,508
3,385
Yes, your assumptions are not quite right.
The wire-wire capacitance contribution from the capacitance to ground is 1/2 of the wire capacitance to the shield (ground) since the capacitance from each wire to the shield are in series (draw the capacitances of the wire to see this).
Thus the characteristic impedance would be √[.18μH / (27pf +1/2*49pF)] = 59.1Ω (close to their value of 58Ω).

3. ### kubeek AAC Fanatic!

Sep 20, 2005
4,691
806
I don´t see any specs of that kind, can you post a datasheet?

4. ### crutschow Expert

Mar 14, 2008
13,508
3,385
Click on the Properties box on the referenced webpage.

5. ### Gibson486 Thread Starter Member

Jul 20, 2012
199
12
Thanks....it is still a little off on other cables, but I guess that means sqrt of L/C is really just a ballpark estimate?

6. ### crutschow Expert

Mar 14, 2008
13,508
3,385
I would think being within less then 2% of their value is better than "ballpark".
The equation √(L/C) is for an ideal lossless line.
There are likely some factors in the real cable that make its impedance slightly different from the ideal.

Last edited: Oct 9, 2015