Click on the help button (+) when in the configuration page. (Or go to http://1.2.3.4/help_adv.html#02 - replace 1.2.3.4 with the correct ip)
This sentence is from hardware versions B2,C1 and C2 and may be different for other models...

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Antenna Selection
The antennas on DWL-900AP+ can be selected for different usages. For most indoor usage, the default Diversity Antenna setting is recommended. If you want to replace one of the antennas with an outdoor external antenna, choose Left Antenna or Right Antenna to maximize the transmit power to the specific side. Note: Right Antenna is External antenna of DWL-900AP+
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Regards
John Burns