Help(aperature coupled microstrip antenna)

I want to improve the front-to-back ratio and also the minimun gain,can anyone give me some sugestions? Thanks.
I tried to change the microstrip feed line to stripline feed( the metal ground on the bottom can reflect some back radiation waves), but I am not very sure about the port setup for stripline, who knows about this? Thanks again.

try to build cavity around the patch like this[/img]

The port setup for stripline should extend to the top and bottom grounds, and should be wide enough so that the fields from the trace do not attach to the sidewalls of the port.

For HFSS models, the recommended port width is 8*w if w>h and 5*w if w<h. Make sure that the port width isn't greater than lambda/2 in any direction or a waveguide mode may be set up as well.

Gain and F/B ratio of a microstrip patch are functions of the groundplane size (the larger the GP, the higher the gain). Also make sure you are using a low-loss dielectric material to minimize any efficiency losses in the substrate, which will reduce antenna gain.

Stacking additional parasitic patches above the fed patch will increase gain as well as bandwidth when designed correctly. This is similar to the concept of adding directors to an endfire array.

You could look into electromagnetic band gap (EBG) structures for the substrate which suppress surface waves and therefore increase radiation in the broadside direction.

And lastly, annular ring loading for circular patches is a method to reduce surface waves and therefore increase the antenna gain. You can find more info on this technique by searching Google.

Good luck.