How to reduce side lobes levels at 60 GHz of Aperture coupled antenna...?

I have designed a 1x5 element aperture coupled array antenna at 60 GHz. I want to have side lobes levels at -15dB from main beam.. How can I achieve this..Theoretically the separation between elements should do the job but I have tried separation between elements from 0.5--0.9 Lambda and still no suppression of side lobes..
Any ideas people..

Awaiting positive reply,
Regards