Thank you so much for your reply. I got 3.5Ghz bandwidth for one element. However, I am not sure is it enough or not?Depends on feeding network. You can achieve >3GHz for single element with magnetoelectric dipole antenna (me-dipole slot antenna) or aperture coupled patch antenna.
thank you so much for your explanation and the Vivaldi paper. I studied it. It has a good bandwidth. However, I am trying to get a good bandwidth from aperture coupled feed patch antenna.Usually the array antenna bandwidth is not much greater than the bandwidth of a single antenna element.
Frequency bandwidth of an array is affected by many factors, as: change of element input impedances with frequency due to element coupling and due to beamsteering, by change of element spacing in wavelengths, and also by feeding structures used.
I think that for your bandwidth requirements a Vivaldi array would be a good option: