The Phoenix connector referrs to the RF connector with the 3 screw terminals on it which you will find on the MRF-300 MRF-350 and MSC-400 as well as the RFX-250 antenna.

Mitch57:

What is really being addressed in this thread is using one or more antennas for a whole house solution and using cat5/6 to run antennas long distances.

We are not necessarily talking about using cat5/6 in place of the cable that comes with the MRF for use in a cabinet, but if you are having trouble over a short run like that you may want to try shielded wire as Jon says above. If it helps please post it here so we can all learn from your experience. If we find shielded wire is better I'm sure URC would listen to us and consider/test our findings.