I recently tried to select a single antenna on AR9300 and it works for
30 seconds only. The subsequent calibration makes the RX signal level
to drop from the usual -30/-40 dBm to -70/-80 dBm, and the
transmission practically stops.
With the attached patch it works, though selecting the antenna doesn't
seem to have any visible effect, at least with "iw wlanX station dump"
(perhaps it works for TX).
I'm using ad-hoc mode:
iw dev wlan0 set type ibss
iw phy phyX set antenna 2