According to what I have read, if you take two identical multiple element yagi-type antennas or better yet, perhaps use log periodics and STACK them one on top of the other in the vertical dimension, with a critical distance apart from each other, you will gain more than 5 dB in overall signal in so doing.
I believe that this is well documented in the pages of Audio Magazine and can be found perhaps in the Radio Amateur Handbook.
This might be of use to you, but you need TWO IDENTICAL antennae!
Find more on the subject over here:
http://72.14.253.104/search?q=cache:QkY9BIfBuJIJ:www.kyes.com/antenna/stacking.html+stacked+FM+antennas+and+gain&hl=en&ct=clnk&cd=6&gl=us
Richard Links
Berkeley, CA