# Three wire parallel transmission line (TDR)

I have a three wire parallel transmission line with a length of around 1000m. The wires are really close to each other. I want to sample the response of the transmission line for two different pulse shapes.

Wire 1 will be used as the common ground. In wire 2 a fast needle shaped pulse will be fed in with a pulsewidth of around 1µs. In wire 3 a square wave pulse will be fed in with a pulsewidth of around 100µs.

When the signals are fed into the wires at the same time, how will the signals affect each other because of the shared ground?

• If I'm not wrong with the math, it will take about 6 microseconds for the signal to get to the end of the line and back. So you have to model waves traveling over the line. Check for software that is capable of distributed parameter line modeling. Commented Feb 5, 2021 at 11:58

You calling one of the lines "common ground" doesn't matter, it carries just as much info as the other two.

I'm assuming "really close" very much means these lines aren't independent; the moment you change the potential field of one, you also change the field for the others. There's no such things as "I feed a pulse into one, but not into the other two", that's literally not how transmission lines work; you've got a field between your conductors, in which the energy is transported, and in a very much "action=reaction" way, you won't be able to tell whether one is ground and the other one is fed without a reference:

Maxwell says your changed potential on one line leads to a current, which leads to a magnetic field around that line; the other lines are in that field, and ampere's law kicks in and there's a current induced; that leads to …

It's transmission line theory all the way.

Now, your scenario really sounds complex enough to say: The only realistic way to answer this is through FEM EM simulation.

• thank you for your answers. Thats what I thought. I think I will just test it out on the transmission line, I don't have alot of experience with FEM EM simulation Commented Feb 5, 2021 at 11:43
• excellent point in time to change that :) Commented Feb 5, 2021 at 11:50
• can you recommend me a free software for this simulations? I always used trial versions, couldn't find free software Commented Feb 5, 2021 at 12:12
• OpenEMS is nice, aside from more or less forcing you to use Octave as programming interface :) Commented Feb 5, 2021 at 12:13
• thank you I will definetly look into it! Commented Feb 5, 2021 at 12:16