references in slotted waveguide antenna array generating difference beam

can anyone give me some references in slotted waveguide antenna array generating difference beam.

The two patents US4958166 and US5612702 show a way to obtain sum and difference beam with slotted waveguide. (if you have problems to get the full-text of the patents go to IP.com - Intellectual Property Library | Search Patent Database | Non-Patent Literature).
greetings

Thanks for your useful information!