F/b improvement of a 3G antenna

I am presently working on a 3G antenna (size 50x50 mm). F/b ratio of the antenna only (without reflector) is around 5dB. I have tried quite a few reflector designs and have obtained F/b not more than 12dB and probably out of ideas now. Can anyone help me out ? The maximum size of the reflector cannot exceed 100x50mm.

The largest size (allowed) is where I would start. You might consider chokes on the edges and/or back.

Have you considered RAM materials?

How about an antenna with a cardoid pattern to start with?

what exactly do you mean by chokes ? is it a spiral coil and how am I supposed to connect the same ?

can you elaborate a little more on RAM material and how do I design an antenna whch gives a cardioid pattern ? Can the shape of the radiating patch affect F/b ?