Is this any CST problem? or setting I have to change for higher frequencies.?
The biggest error source in EM simulation is the
user, not the software. If you just used the defaults, results can be good or bad. Nobody knows, because every EM model is different. You need to learn how to use your EM tool, and what good settings are for your models.
You need to understand why measured and simulated don't agree (enough) to make progress in your work. You define what is good enough, because only you know what the design requirements are.
but to send the work in any journal I need to show the good match between the simulated and measured results
To publishing the work in a journal, you don't need simulations. What you need is some
new scientific work that was
not already published by others. If it's
relevant and
new, and measurements show good performance, go ahead. Even if you have measurements and simulations that don't agree, and you can
explain why there is a difference, that's ok.