From (http://www.cisco.com/univercd/cc/td/doc/product/wireless/airo_350/350brdgs/brhig/br350apc.htm#xtocid8 Antenna Basics):



A diversity antenna is physically two antennas (sometimes in a single package) that provide diversity antenna operation. In diversity operation, the radio receives signals on both antennas but responds (transmits) on the antenna with the strongest received signal. Diversity operation helps improve system performance when signals are being reflected along different paths to the antenna. Diversity antennas for the bridge are available in 2.2 dBi and 5.2 dBi gain ratings.