How to reduce sidelobe in Waveguide Slot

Here I have waveguide slot with 64 slot, with matching in working frequency but has large sidelobe (according to my lecturer), that's 8.6 dB. oke i wont ask whether it's normal with 64 slot or not, because my lecturer insist me to reduce it. I've read many journal and aplicate it, but it's not working to my simulation design (in CST software). Please help me to reduce this sidelobe. what factor that affect most in this antena design? Thank you.

You have to change amplitude distribution of the slot array and chose the optimum one you need.
so you have 2 ways:
1. Changing length of the slots
2. Changing distance of the slots to the center of the waveguide.
Read something canonical about AR, like Hansen et al... Key words are uniform, Dolph-Chebishev, cosine on pedestal, Gauss and others distributions.

thanks then, yeah i was thinking i need to learn about chebyshev. do you have any refrences? i'm searching..