AWR has one of the nicer free transmission line loss calculator apps 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:
Obviously TxLine can be used in a host of other ways, this just shows one example.
The TxLine is a free download from Cadence/AWR. Web-based and easy to use.
Update: If you really want to do this, better use the coupled microstrip model in the loss calculator instead to get the right loss value to compare to the differential loss plotted from measurements. You get the idea. Not hard to do – just a bit different.