Questions about microstrip antenna array~

I designed a 4X4 microstrip antenna array, from the simulated result the gian is 18db and it is just so-so,but the central frequency has a big offset (from 12.5GHz offset to 11.5GHz and the S11 is also worse). I have attemptted to change the distances between the elements but I still can not get a satisfactory result. Who can tell which resaon can cause this problem? Thanks~

Try to understand what you have built (that is knowing the behavior of the radiating structure and the feed network inclusive power dividers). Look to current and/or voltage distribution whether it is as you expected. Mostly some scaling and/or change in trace width (that is change in characteristic impedance of transmission line sections) will do the job when it is just a matching issue (and other things are OK).

Changing distance between elements will have effect on S11, but will affect radiation pattern (and gain) also.

hi
pls attach your antenna with out antenna it hard to judgement and for increase your bandwidth better you change first antenna