Take your coax loose from the isolator. Get some kind of continuity device
(VOM / battery and bulb / etc) and check to see if your coax is shorted.

If so, there's your problem. Then you can work on finding the short. Each
time you do something, check continuity again to see if what you did clears
the fault.

If your coax is OK, then call the folks who made the antenna. If it's Radio
Works, they're very helpful. They can talk you through some simple things
to try to further troubleshoot.