Installed a Diamond NR770HBNMO on my Jeep Wrangler yesterday and I'm seeing SWR of 2.5:1 on 146.520 with my Kenwood TM-V71A. Most antennas for handheld(HT) and mobile UHF/VHF ham radios do not require tuning so I'm puzzled why the reading is so high. I'm using about 15 feet of RG58 coax to reach from the spare tire mount to the radio. Ground plane should be decent with the metal body.

Before I start cutting anything, what should I check first? The antenna was supposedly pre-tuned from Diamond for 2m/70cm. Using a Surecom SW-102 SWR meter.

You SHOULD NOT transmit with your CB at SWR levels above 3.0. If your SWR needle swings all the way to the right (off the charts) when getting your 3.0+ readings, you almost certainly have a major installation problem. This is almost always the result of a poor ground or incorrectly assembled stud, but on rare occasions can indicate a faulty coax, antenna, or incorrectly attached SWR meter. Check your connections first - make sure the coax connectors are tight and properly assembled. Also verify you're measuring in a clear area away from buildings and other antennas.

A short in the coaxial connectors may also be the culprit behind abnormally high SWR readings. Issues with the coaxial cables are often identifiable by eye, such as severe bends or pinches. You should know that it's essential to use 50-ohm coax for single antennas and 75-ohm for dual. When all else fails, sometimes it's necessary to replace the coax cable because there's a failure inside the line. RG58 isn't the greatest for VHF - consider upgrading to RG213 or LMR240 especially with a 15-foot run.

Had the same issue with my Diamond antenna on my JK! Turned out the NMO mount wasn't making good contact. I had to sand the paint around the mount area to get bare metal contact. Why does my SWR change after raising the antenna? Nearby objects and ground effects change electrical length and impedance. Also try moving the Jeep to a different location for testing - sometimes nearby metal objects can throw off the readings.