Thanks all for responses. I did have a short when I first installed, but fixed that soon after.
One thing I did was when I was annoyed with it not working and the short, I cut approx 200mm off the end of the coax. Could this be the cause?
Lotz-a-landies..How do I test centre pole at both ends? continuity test both ends..radio end and at antenna base?
Is there any protocol for testing a UHF radio...channel to use?

