That Ringo Ranger will normally show a short is measured at the end of the feed line, or at the input of the antenna, it's normal. If you take a look at the matching system, that ring, and trace out the connections, you'll see they are a direct short. That's at DC, but not at AC, and all RF is AC. Your coax isn't pinned, and I doubt if there's anything wrong with it.
If there's a change in the impedance it's usually for some other reason dealing with the antenna, or feed line, or both. Something is changing physically. Best thing would be to take the antenna down and make sure it's all still there and still assembled correctly. Remove the feed line and check for it being shorted, or opened. It should show that it's 'open' if removed from the antenna. If it isn't, then you've found the problem. If it isn't shorted, then take a very good look at that antenna, that's where the problem may be. Anything loose, not like it was originally, anything changed? Also 'walk' the feed line to see if there's any pins, or other damage. Does it move a lot, check for broken places, anything different.
My money would be on the antenna having a problem of some sort.
- 'Doc
(In 40 some years, I've only seen coax 'pinned' once. It was mine, and I had done the pinning for a particular reason.)
