Ubiquiti airMax Sector AM-9M13-120 Quick Start Manual page 6

5. Attach the U-Brackets to the antenna:
a. Secure the Slotted U-Bracket to the upper Mounting Lugs
of the antenna using two Serrated Flange Nuts.
b. Secure the other U-Bracket to the lower Mounting Lugs
of the antenna using two Serrated Flange Nuts.
Note: Orient both U-Brackets as shown below.