Characteristic Impedance of a cable calculation

crutschow

Joined Mar 14, 2008
27,395
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?
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).
 

Thread Starter

Gibson486

Joined Jul 20, 2012
336
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?
 

crutschow

Joined Mar 14, 2008
27,395
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?
I would think being within less then 2% of their value is better than "ballpark". :rolleyes:
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.
 
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