How do I simulate the coupling between two adjacent transmission lines on a PCB?

Simulating the coupling between two adjacent transmission lines on a PCB using an EM solver determines the crosstalk magnitude, coupling direction (forward vs. backward), and frequency dependence, enabling the designer to verify that the coupling is below the acceptable threshold for the system (typically less than -20 dB for most RF designs, less than -40 dB for precision measurement applications). The simulation setup is: create the geometry (draw the two parallel microstrip or stripline traces with the specified width, spacing, and coupled length; include the ground plane(s) and substrate stack; the traces should extend beyond the coupled region on both ends to provide a measurement reference), assign ports (place a port at each end of each trace (4 ports total for a 2-line coupled structure); the port numbering convention follows the standard directional coupler convention: Port 1 = input, Port 2 = through (same trace, far end), Port 3 = coupled (adjacent trace, same end as input), Port 4 = isolated (adjacent trace, far end)), run the EM simulation (simulate across the frequency range of interest; the simulation produces the full 4-port S-parameter matrix), and interpret the results (S21 = through (insertion loss of the main trace), S31 = backward coupling (crosstalk at the near end), S41 = forward coupling (crosstalk at the far end); for microstrip: forward coupling dominates at higher frequencies (because the even and odd modes propagate at different velocities); for stripline: backward coupling dominates (because the modes propagate at the same velocity). The coupling increases with: narrower spacing (exponentially sensitive to s/h ratio), longer coupled length (proportional for backward coupling, oscillating for forward coupling), higher frequency (dielectric and magnetic coupling both increase with frequency), and thinner substrate (the fields extend further laterally when the substrate is thinner relative to the trace width)).