Hi Paul,
I started off with a Radar-Rarma fitted outside,it did`nt last more than approx 6 months,rusting & reception dropping off due to the deteriation of its condition.I mentioned this to the owner of a local ham radio store who said he thought it was an indoor aerial,although I dont think it states that in the instructions.Ref the coax always go for the thicker top end stuff,preferably Westflex 103,or RG213.Use N type connectors rather than PL259`s if poss,the N type are far better matched to the 1090mhz frquency.It also depends on how long your cable run will be,if the aerial is going in the loft with a short cable run you might get away with a thinner & much cheaper 50 0hm coax.
My set up now is a DPD 1090 antenna,60 ft high, N connector onto the aerial,20mtrs of westflex 103 to another N connector with an N to BNC adaptor then straight onto a JIM preamp then onto the radar box with an SMA to BNC adaptor.
Hope this gives you some ideas,no doubt some other forum members will also give you some useful advice.
Regards Paul,voyager8.