wideband patch antenna design

hello,
I have to design a patch antenna with 25% bandwidth. I would like to use stacked patches (possibly two) with coax feed, can anyone suggest some design rules or design formulas to start with, instead of leaving everything to the optimiser of any EM software. Any reference to papers, books, etc is appreciated.
thanks in advance for any help

quick google search turned up a bunch:
I haven't verified the content of the paper but sounds like what you are looking for.
http://ieeexplore.ieee.org/xpl/login...umber%3D817653

http://athena-gatech.org/papers/rlli11.pdf