Yes the antenna is a POS and uses the coax as part of the antenna. At 4ft. It isn't much of an antenna on 27mhz. Comparing it to other antennas which are closer to 108" or 9ft. With that said, Yes if you cut the calibration back 1/2 it's going to show 1/2 of what it should. You have grounding issues, a bad mount ground and a mediocre antenna at best. Check all grounds at where it's mounted. Make sure that the area where the mount is has all the paint removed and use some no-ox if possible on all terminations. Check coax for a short. Unplug it from both the antenna mount and the radio and use an ohm meter. Between the center pin and outer jacket there should be no conductivity. Check that there is also good continuity from center pin to center pin and outer shell to shell as well. A good sirio performer 5000 mag mount on the roof would probably be a better choice. The magnets have a nice rubber boot as to not scratch paint. That or drill a hole in the roof and buy a Breedlove mount and a good tall antenna. Anything over 6ft is going to work way better than what you have now. The antenna you current have on there like I said is a mediocre one at best and will not have good rx and tx due to its length. But like I said, first check all connections and the mount itself. Good luck and hope you get it resolved.