Thank you JohnJoe:
Meanwhile looking for a feedback on the topic I reasearched more and found that, indeed the Ar filter is used when having resonant structures. And probably I have to use it since I am simulating CBCPW at very high frequencies (up to 300GHz), where finite ground widths can transform into patch like antennas and resonate. I know that best way is to use VIAs to shortcircuit parallel plate mode, but I am doing some trials in order to evaluate the ground width. I am trying to optimize the top GND width such that the first higher order mode has the cut-off frequency very close or beyond my highest frequency of interest.
The result curves I have in S-params are typical for a 50 Ohms T-line, with the difference that these results vs. S-params (AR) have some ripples (like small oscillations), and from what I was able to document myself this is cause of some resonances taking place. And it's exactly what's happening with ground widths comparable with Lambda/2 at the operation frequency. Looking at S-params(AR), they are ripple free, and I believe this is the more accurate frequency response!?! Please correct me if I am wrong.
Another aspect is the "Steady state criterion". In some cases (when the finite GND widths are comparable with lambda/2) the criterion is not met, but in the end I managed to avoid this warning and reach the "Steady state criterion" threshold by increasing the number of pulses in the Steady State window from menu Specials in Transient solver (default is 20 pulses, I raised it to 60 such that energy balance occurs and steady state criterion is reached).
If anything of what I said is wrong please correct me, as I am new to CST! Wainting for a feedback.
Many thanks,
George.