How do I Analyze Crosstalk in an Unbraided Multi-Cable?
How can I study crosstalk in a pair of straight wires that share direction of current, but with varying phase*?


You can estimate the mutual inductance between an aggressor loop (signal wire + return wire) and a victim loop (signal wire + return wire). Given that and the current signal risetimes and amplitudes you can easily calculate the crosstalk effect on the victime wire.

If the two loops share the same return path, you should also consider IR drops on the return path due to the aggressor signal as a crosstalk on the victim channel.

Edit As Andy points out, there is also capacitive (electric field) coupling to consider. AFAIK, this is rarely the dominant source of crosstalk in the type of cables you seem to be asking about, although it could become an issue if the cable and/or its terminations are badly designed.


  In extension to "straight", these types are also of interest...


Parallel wires is going to be the worst case scenario. All of your other scenarios are going to have lower crosstalk than a parallel pair separated by the smallest amount of separation in a non-parallel pair.

Wires crossing at 90 degrees will have the least  crosstalk.