The Ubiquiti SR-2 is a high-power 11bg card based on the Atheros AR5213 MAC. It has three antenna fittings - one UFL, one MMCX, and a third set of solder pads for a non-existent connector. The first two are treated as separate antennas and are controlled by an external antenna switch. The third looks like it's an alternative to the UFL connector.
I discovered that these cards occasionally calibrate their NF at some impossibly low level - under -100 dBm. The reason? Antenna diversity is occasionally selecting the MMCX connector as the RX antenna - and this never has an antenna attached. Tsk.
Antenna diversity also often chooses the MMCX antenna for TX. Grr.