Note that at the time of writing this article this information is captured incorrectly in the data sheets. The following minimum preambe length requiements are needed for packet loss free reception and no sensitivity loss compared to non antenna diversity applications (less the the insertion loss of the T/R switch and the longer traces obviously).
When AFC is not enabled antenna diversity requires 80 bits of preamble.
When AFC is enabled antenna diversity requires 88 bits of preamble.
Si446x revC2A and A2A devices support antenna diversity with DSA. This operation requitres 64 bits of preamble. Note however that co-channel rejection with DSA in antenna diversity mode is poor so this configuration is not recommended.
Leave all the antenna diversity parameter calculations to WDS. Make sure you transmit the minimum length of preamble and Rx will just work.