couple between two horn

could anyone help me to solve the following question:
my two horn is ultra-broad band ,that is 4GHz~18GHz.when they work together ,the capture distance of the two horn is about 9mm .
so the couple of the two horn is too much .
especially in the lower frequency band .
what shall i do ?could you give me a hand

just put absorber plate between the horns and few cm in front of the antenna

http://www.eccosorb.com/catalog/eccosorb/index.asp

pl