Hi!
I've designed a 4 element microstrip array antenna at 3GHz. But I got a return loss of -24dB at 2.6GHz instead of 3GHz. Why is that? and What do I have to do to achieve -24dB return loss on 3GHz? Can anyone help me?
Coupling suppression is still an active research area, as there's currently no great solution. Search "Electromagnetic Bandgap Structures" or "High Impedance Surfaces" for coupling suppression to see some of the more mainstream approaches.
Honestly, if you're new to the field, optimizing the patches in the array may be a more simple approach.