Not any AC specific "AC antenna" - didn't say that, but many users are not aware of the importance of Port to Port isolation for good MIMO performance. With AC and 256QAM, its even more important. Flashnet in Austria (WISP) have done a lot of testing on AC, and can probably provide more on hard facts and results. AC with 25 dBi integrated alu enclosure - I'm sure you can find it...
/TB