calculation accuracy in HFSS

I seek to determine the parameter S21 of a structure for a fixed frequency and a horizontal axis variable. The curve obtained for S21 is very unstable. How to increase the accuracy of the calculation of S21

thank you for your help

In projects tree find Analysis, double click on "Setup", increase "Maximum number of passes" and decrease "Maximum Delta S"

Thank you for your help

I increase "Maximum number of passes" and I decrease "Maximum Delta S", but I still have the same problem of instability of parameter S21

The second problem: the S21 parameter changes by changing boundaries of structure. Can you give me a method that allows me to calculate the exact dimensions of the boundaries of structure

Sometimes, you need to increase the input and output coupling strength to get a stable s21 curve. If the coupling is too weak, you won't get stable results.

How to increase this coupling knowing that both Ports 1 and 2 are of type excitation lumpPort

How "instable" is the S21? Varying between -10~-5 or -25~-20 are different stories.

IF you have Transmission lines that are very close to the boundary, your results will differ with altering the distance between these lines and the boundary.
Increase this distance, so your results will be accurate. this might be your problem.