waveport impedance

This is for the purpose of PCB structure simulation. I saw a few recommendations of wave port size for microstrip line. But I still tried to do port only sim by varying the size of port. i expect to see that impedance change becomes minimal after port size gets large enough. however, the impedance either goes higher or lower as width increases to be very large. any explanation for this?
-------------------single ended MS
Width Height Z0
20 24 47.808
40 24 50.036
60 24 49.415
80 24 49.58
100 24 50.355
120 24 46.275
140 24 49.038
160 24 50.03
180 24 48.184
200 24 49.541
-------------------diff MS
Width Height Zdiff Zcomm
30 40 85.947 29.65
40 40 85.257 30.066
60 40 83.354 30.124
80 40 83.735 29.829
100 40 83.025 29.699
120 40 78.254 29.542
140 40 75.978 28.34
160 40 78.037 28.151
180 40 78.765 30.257
200 40 82.719 30.17

main thing is value is converging to some value deviation is not much...it is worrying if variation in imped is much with dimensions of port.


u can check S11 also and see whether its converging or not