Hi
At least one legacy rates (802.11b/g in 2.4GHz and 802.11a in 5GHz band) to be enabled in order to achieve 802.11n rates.
So your choices are to disable low data rates (1,2,5,5,6,9,11Mbps) in 2.4GHz band as are using 2.4GHz band for your bridge. Below is antenna spec which you are using.
http://www.cisco.com/c/en/us/td/docs/wireless/antenna/installation/guide/ant2480v.html
HTH
Rasika
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