ADS and CST same operating bands with different values of S11

Dear all,
I would like to know when we can say that we have good agreement with obtained results of an antenna an its proposed equivalent circuit one is simulated using CST and the other one using ADS, Knowing that I get the same operating bands but the difference between the two curves of the reflection coefficient is around 20dB
Thanks in advance

If you have a difference in results, there might be something different in the models.
Or you might use the wrong metrics for comparison. For a very narrow band antenna, a small shift in resonance frequency can cause a large difference in S11 if you compare at one fixed frequency.

thank you for your reply, please allow me to reformulate my question, what are the tolerances for which we can say that we have a reasonable agreements for my case I get the same operating bands for both ADS and CST but with a lower S11 for the CST ,i.e. S11(CST) < S11(ADS), the difference can be in the scale of 20dB for same resonances.
thank you in advance for your help!

The difference is in how you set up and define the port. S11 is a measurement of how much energy is reflected back. So in one sim, you have a good impedance match at the port, and in the other you don't. Check your port definitions to make sure the impedances are the same, say 50ohm or whatever. I think CST has a checkbox for matching port impedance during simulation setup. Else you might be going from freespace 377ohms to whatever the impedance of the antenna is, resulting in a poor S11.

Your question is too unspecific to answer.
The 20dB difference between S11=-10dB and S11=-30dB is bad agreement.
However, the 20dB difference between S11=-30dB and S11=-50dB can be considered excellent agreement.