I would suspect the plastic / uhmw washer is missing or the antenna is not installed right. Start by looking at the mount and make sure the center stud is not grounded. This can happend if the insulating washer is missing, or some extra parts were used and shorted the antenna to ground. On a side note I had a smaller fiberglass CB antenna that was right out of the wrapper was not cut anywhere close to the right length, and created high SWR.
For CB antenna's toss those fiberglass sticks and spend a couple more bucks for a nice Wilson antenna. Why? I find for mobile use the heavy fiberglass sticks are a pain. You need an extra heavyduty mount, which mean it will be on the bumper or edge of the bed. I love my base loaded steel whip.
You will need to post pic's as I have to aSSume what you have, and how its install. Also damaged coax will create the same issue. Try a mag mount antenna to see if it has the same SWR
