Hi Matthew,
The 2nd and 3rd Antenna connectors for 2.4 are used to support (MRC) and
it is recommended to use these
The 2.4-GHz radio performs Maximal Ratio Combining (MRC), which takes advantage of multipath signals received across up to three antennas to improve signal quality.
The 2.4-GHz radio can operate with fewer than the listed number of antennas. However, optimal performance is achieved when using the maximum number possible.
Moreover, using less than the total number of antennas supported leaves the unused antenna connectors exposed to the weather; they will need to be covered.
http://www.cisco.com/en/US/prod/collateral/wireless/ps5679/ps8368/product_data_sheet0900aecd8066a157.html
Cheers!
Rob
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