[CST] Luneburg lens antenna design

Dear members
I would like to design a Lunebuerg lens on a Tri-band antenna to improve its gain. Could someone suggest and share some IEEE papers/Books to start working on the lens design and understand the basic principle of it. I am using CST MWS for antenna design. Kindly also suggest on how to design the lens in CST on a tri-band microstrip patch antenna?
Awaiting replies,
Regards

Rozendal Associates is a source for lenses. You might contact them for advice. Maybe they have useful white papers available.
Azulykit