I'm trying to model a differential digital signal going through 4000 feet of twisted shielded pair. The nominal impedance is 120 ohms and capacitance per foot is 14.6 pF.
For 1000 feet I calculated the inductance should be .0002 H and 14600 pF. I broke the model up into 4 segments. Each one with 14600 pf cap, and .0001 Henries on each wire. I put four of these together with a 120 ohm resistor termination.
The results look close but not exactly what I see in reality. The pulse width is really wide compared to what it should be and the output is ringing more than it should.
How many segments is enough to break 4000 feet into? 10? 100? Is there anything else I can do differently?