The XM+ from FrSky is a diversity 2.4GHz antenna chip in their new line of micro receivers. Supporting 16 channels over S-Bus compatible with D16 mode. The diversity antenna system gives you full range while still only weighing in at 1.6g and measuring 21.5 x 12 mm. This chip offers a full operating range in an ultra-tiny package.
Also check out the XM which is a single antenna receiver that is slightly smaller for ultra-micro builds.