Stuart,
We generally do not recommend peripheral components for our units, we give a spec that they need to meet and leave the choice to customers, this is for lots of reasons, the main one being we are not antenna designers, it is a black art and every application is different.
You really need to go to a specialist antenna manufacturer who will be able to meet your needs either with a COTS unit or something custom, most of our partners have options to offer.
Regards
Matt