AWR have one of the nicer free transmission line calculators I have seen in a long time. Highly recommended.
Recently I needed to approximate a cable with the following insertion loss for a 25 cm long strip:
The approximation was required for an IBIS simulation in SigExplorer, so a microstrip model would be fine – also this is pretty similar to the physical buildup of the actual cable.
A reading from the insertion loss graph shows ~5dB at 3GHz. For a 25 cm strip, this is about ~20dB/m. Using the free TxLine from AWR, the following possible set of reasonably closely matching parameters were easily found:
The phoney physical characteristics could then easily be added to a microstrip model in SigExplorer, to get a rough approximation of the cable behavior in terms of insertion loss.
Obviously TxLine can be used in a host of other ways, this just shows one example.
TxLine is a free download from AWR.
Update: If you really want to do this, better use the coupled microstrip model instead to get the right loss value to compare to the differential loss plotted from measurements. You get the idea.