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Has Anyone Been Able to Simulate a Twisted Pair Cable? - RF Cafe Forums

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Post subject: Has anyone been able to simulate a Twisted Pair Cable? Posted: Tue Sep 20, 2005 3:27 pm
Good day. Has anyone been able to simulate a twisted pair cable with ADS, Microwave Office, Eagleware, etc.?

I am trying to examine the effect of different twists and how they would affect coupling between the two wires.

Thank you for your time and help.


Post subject: simulation of twisted pairPosted: Wed Sep 28, 2005 5:56 pm
Because you're dealing with the physical configuration, you can't use a simulator which isn't physics-based for your task.

Sounds like you need an 3-D electromagnetics simulator, if you want to investigate this in detail.

But - the largest effect will be the increase in wire length to cover a given distance. You should be able to calculate this easily.

The second-order effects will be the increase in capacitance and inductance.

It may be that you don't need anything other than the largest effect.

At any rate,
Good Luck!


Post subject: Re: Has anyone been able to simulate a Twisted Pair Cable?Posted: Sun Oct 02, 2005 6:48 pm


Joined: Sun May 09, 2004 1:23 am
Posts: 10
Location: Morgan Hill, CA (Silicon Valley, Bay Area)
Michael wrote:
Good day. Has anyone been able to simulate a twisted pair cable with ADS, Microwave Office, Eagleware, etc.?

I am trying to examine the effect of different twists and how they would affect coupling between the two wires.

Thank you for your time and help.

What exactly are you looking for when comparing different twists in a twisted pair cable? A 3D simulator might help, but I would think that you'd just get different impedances for different twists and that would be about it. Will the twisted pair be kept far away from metal objects? That would make a difference too since a twisted pair isn't shielded like coax is.



Post subject: Posted: Thu Oct 13, 2005 7:12 pm


Joined: Fri Sep 02, 2005 7:25 pm
Posts: 34
Location: Hampshire UK
Twisted pair like telephone wire pairs is around 110 ohms.
The impedance of shielded twisted pair is not much different to the shielded variety. Cat5 network cable amounts to a relatively high loss, but useful cable for short range applications, is of this type.

You can get individually shielded pairs cable. In all these, the presence of the shield is to protect the cable from external interference fields and to limit the amount of conducted and radiated EMI from the cable. The shield does not primarily define the impedance, which is a function of the conductor diameter, the spacing, the dielectric constant of the plastic separating the conductors. It would affect it a little.

The "shield" around coax should not be thought of as a shield. It is one half of an unbalanced transmission line. It is what happens if you think of one of the conductors of a balanced pair getting bigger, and morphing its shape (in cross section), to surround the other, to eventually assume the shape of coax. The lines of electric field then become radial between the centre conductor and what now surrounds it. This coax form of outer very definitely has a profound effect on the impedance, by its dimension and the dielectric within.

Grounding the outside when there is a standing wave on a coax cable is a major sorce of EMI trouble. Triax, which is a coax with a second outer braid, allows the outermost braid to be grounded (usually one end only), and is an (expensive!) shield - the coax equivalent of the shielded twisted pair.

In theory, if one conductor of a twisted pair gets up against a metal object more than its partner, you get some unbalanced current. In practice, with a pair having a high rate of twist, it is remarkably immune to coupling. I once tried out some HF cable distribution stuff that had six pairs around a mandrel plastic, in a jacket that was carbon loaded to make it "pre lossy" so that it did not change its characteristic much as it aged in use. Provided all the unused pairs were terminated, the balance between pairs was better than -60dB. We could run a small electric drill on one pair while sending a vestigial sideband TV signal up the cable the other direction. Definitely a "don't try this at home" stunt!

Then again, it only takes ONE leg of ONE pair in a fat cable to get disconnected to have all the rest get mangled with crosstalk! Ask any telephone engineer.

Posted  11/12/2012

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