FYI in /tmp/system.cfg (configuration file):
radio.1.txchainmask=[bit mask]
radio.1.rxchainmask=[bit mask]
Where bit mask means which chain/chains to enable:
1(0bit) - chain0
2(1bit) - chain1
4(2bit) - chain2
So you can make combinations like 1 and 4 = 5, means chain0 and chain2 enabled. And so on.
NOTE: Chains are HW specif, not all have all chains or the same chains.
I guess my sample will help to make live easier and avoid custom scripts to be installed.
Lifted from: http://community.ubnt.com/t5/Installation-Troubleshooting/iwpriv-to-make-2x2-devices-work-like-1x1/m-p/657119/highlight/true#M57052