Why Resonant frequency is being shifted when solution frequency shifts in HFSS?

hi
I simulated a structure with a solution frequency 915 MHz, which was desired frequency.
I repeated simulations with solution frequencies 2 GHz and 2.5 GHz.
Their S11 results were different from each other. The resonant frequency shifted each time to right hand side of x-axis.
So What should I set the solution frequency in order to achieve correct resonant frequency ? Should it be The desired frequency or end of the frequency range I use in my sweep ? if it should be end of the range, so how can I get the radiation pattern for my desired freq which is 915 Mhz ( since radiation pattern are illustrated for solution frequency)
Solution type is Driven Modal

My blessing to anyone who can help me, or at least tries to ...

Regards
Iman

Salaam Iman

if i was you i would do a few things myself first.

check if my solution data has converged

use adaptive mesh, the difference in the resonances will be mainly due to impedance mismatches.

the solution frequency should line within the frequency sweep range and around the passband.

or else post your file here i will be able to look into and give you a feedback

Regards