How do you calc Characteristic Inpedance & C ap of a cable?

[ariccardo]

Can someone please tell me how I can calculate the characteristic impedance and the shunt capacitance of a cable, and what variables are required to do so or is there a website with formulas? An exampel of what I am lookign for is I need a cable with 16 pF shunt capacitance and 120 Ohm charactaristic impedance how do I calculate if a cable is so wihtout data sheets?

thanks

 

[VEBill]

You have to look it up on a data sheet for the cable type number, or measure it.

The characteristic impedance of (for example) a coaxial cable is a function of the geometry (outer diameter of the inner conductor and inner diameter of the outer conductor), but the dielectric constant of the insulations enters into it - so you're back to measuring it again. I suppose if you recognize the insulation material, then you could try to work it out based on measurements. But if the insulation is foamed, then you're back where you started.

The formulae are here:

http://en.wikipedia.org/wiki/Coaxial_cable

Whatcha up to? Perhaps there are other approaches...

 

[VEBill]

Ah, this is related to your RS485 cable quest.

So twisted pair, not coaxial.

Amusingly, Googling 'twisted pair characteristic impedance' - the top two hits state: 1) "Note that no twisted pair telephone line has a characteristic impedance less than 120 Ohms at any frequency." and 2) "Typically, twisted-pair telephone cables have an impedance of 100. Ohms above 1 MHz." An amusing contradiction.

 

[VEBill]

http://www.epanorama.net/documents/wiring/twistedpair.html

All the examples listed (in the first bulleted list at the above link) are between 100 and 150, or higher. The point being that 300 ohm twinlead was commonly used for TV antennas. If you've ever seen such 300 ohm twinlead then you'll know how far apart the two conductors were spaced.