The AP-104 has 4 antenna ports; 2 for each radio. They are assigned as follows:
R0/A0 - Radio 0 (5GHz), chain 0
R0/A1 - Radio 0 (5GHz), chain 1
R1/A0 - Radio 1 (2.4GHz), chain 0
R1/A1 - Radio 1 (2.4GHz), chain 1
R0 = Radio 0 (5.0GHz)
R1 = Radio 1 (2.4GHz)
A0 = Antenna 0
A1 = Antenna 1