On second though, those Y adapters are D-channel, aren't they... So that
rules out any linear LNB that I can think of. Would work for DishNet LNBs I guess. Although I'm not sure why you would want to run DishNet from a Fortec dish unless you have really poor signal strengths from your current DN dish. Even then, there might be focal length issues -- would be a good experiment though.
kat