I tested the modulator with a short line last night and it works fine. I then went to the halfway point (about 30') and see serious interference already. I think I may have damaged the cable while I was installing by yanking a kink through a small hole. I probably should have straightened the cable better before running it, but I was rushing the job.
I'll need to get more cable and see if the length is the issue, but I suspect this will be fine once I replace the cable...
Nothing earth shattering here, but I did get a 100' RG6 cable and it works fine. I must have damaged the other while I was installing. I'm going to try and tie one end of this one to a tree and stretch it out straight a bit before I start this time around.