50/75 ohm cable measurement? (1 Viewer)

[Jerzy]

Pardon my ignorance, but what defines the resistance/impedance of a given cable? Is it the actual resistivity per unit length or is it something to do with the type of insulation between the conductors. If the first question is the case wouldn't the length of cable make a huge difference?

 

[KurtBJC]

Characteristic impedance is dependent upon (1) the outer diameter of the center conductor, (2) the inner diameter of the outer shield, and (3) the dielectric constant of the material between them. It's not dependent on length. The idea here is that if one were to make a transmission line of infinite length, it would behave as a termination at the specified impedance--so that what this means is that if you terminate the line at any point with a load having that impedance, then you'll get a total impedance the same as the load. If instead you use a mismatched line and load, the total impedance will be neither the impedance of the line or of the load, but some function of the two.

How this works out--and I'm forgetting my formulas right now, so can't give you specifics--is that characteristic impedance depends on the relationship between capacitance and inductance. That's why, for example, you will find all 75 ohm coaxes having capacitance somewhere in the range of 16-21 pF per foot, while 50 ohm coaxes have much higher capacitance.

People get confused about this sometimes because they try to measure it with a VOM--and, of course, the resistance measured by the VOM will be very small (for continuity down a conductor) or very large (for resistance between center conductor and shield) but will never match the characteristic impedance.

 

[Jerzy]

Thank you Kurt. That was the best and most understandable answer to that question that I have ever heard. My electronics experience is more practical than theoretical, as I suspect for most readers of this forum.

 

[PaulT]

Here's a link for your reading pleasure :

http://www.epanorama.net/documents/w...impedance.html