Antenna diversity can be controlled using sysctl (/proc/sys/dev/athX/{diversity,txantenna,rxantenna}):

diversity:
    0: no
    1: yes

txantenna and rxantenna:
    0: auto
    1: antenna 1
    2: antenna 2

Which connector corresponds to antenna 1 and which to antenna 2 depends on the particular radio hardware. For example, to disable diversity and select antenna 1 for receive and transmit you'd do the following:

    sysctl -w dev.ath0.diversity=0
    sysctl -w dev.ath0.txantenna=1
    sysctl -w dev.ath0.rxantenna=1