Like all things "wheeling", installing a radio transmission antenna on a Jeep is a compromise. The compromise is between the best transmitted energy pattern and ability to remain attached through the "thick and thin" of off roading.
The "SWR" (standing wave ratio) is not affected by things placed nearby the antenna--the pattern of transmitted energy is. What is the objective of your "radio station"? You want to set records with how far you can transmit? Or do you just want to talk to your wheeling mates, let's say, up to a mile away? Being able to talk to the world is cool, but the ability to locate the antenna with that objective in mind and KEEPING it attached to the Jeep may be undo able. Defining your choice of off roading, ie: trees, brush, desert, should describe the physical problems will you run into with regard to where and how you should mount your antenna. I first installed a "desert patrol" type mount on the right, rear quarter panel, with an antenna that rose 3 ft above my Jeep. Looked cool, gave me great radio coverage, but "wipped" the bejesus out of the side of my Jeep, and any one unfortunate enough to be standing close by when I passed! Also, it took about 2 encounters with low branches or brush to "relocate" my antenna! Next position was on my spare tire carrier (swing-out kind). It's inboard enough so that dragging through brush won't affect it; it's high enough so I have decent "trail" communications. So far, that's been the best location for my style of wheeling. I've noticed my OEM radio antenna, mounted on the passenger side front fender ('88 Cherokee), takes a beating as well. So I'd be disinclined to mount my CB antenna there. My $.02.