I configured on the table two cards SR71-A one with ap-mode=ap-bridge and another with ap-mode=station in 2,4GHz-onlyg. I got link only with next HT settings: enabled Tx/Rx chain 0, Tx/RX chain 2. As I understood HT settings responsible for switching antenna output. That means that this cards won't work without enabled MIMO? Is the same thing with SR71-15?
Thank you.

Can any answer?

HT settings responsible about what RX/TX path is used. By theory it is enough enable any chain RX and TX and you can make 802.11g connection. In practic I not test this cards yet.

Just only check the side of chains you want to use, and use the desired antenna port (ports on card are labled Chain0 and Chain1. Also, why are you using these MIMO cards for Gonly? If you are using G Only, use the XR5 cards, you'll have more power.

You mean XR2? XR5 not support 802.11g
http://www.ubnt.com/downloads/xr5_datasheet.pdf