Please read post 1 here first:

[Link: remotecentral.com]

If your mx-850 is after Nov 1, 2006 build date you can use the MRF-260, 350 or MSC-400 based upon your needs. I'd recommend the 350 at a minimum due to the remotely located antenna.

If it is prior to that date, go with the MRF-300 (not the MRF-250).