Combatvet is right on the money here, all I would add is make sure you mount and ground to the body not spare carrier and ditch the alloy bracket, look for a quality suitable mount or use half a mirror mount in the same way as Combatvet and clean away the paint to get a good ground. The body of the Jeep also forms part of the antenna so good connections are essential to put a signal out get it wrong and you'll burn out your transmit chips.
i can't tell which make of spare carrier you have but some have PTFE bearings which can insulate the carrier from grounding, a high SWR reading can be either poor grounding, too small a ground plane or both! be sure to use the longest physical length antenna you can and place it as high as you can on the vehicle and as far away from metal mass as possible, yep a struggle on a Jeep I know but these are guide lines so you will have to compromise.
Top loaded antennas will put more of the signal out from behind objects like spare wheels etc so I would use these in preference to center or base loaded types. I don't know if your aware but a full sign wave at 27mghz is about 34 feet end to end, antennas are divisions of this length the shortest useable length is 1/4 wave or about 8'6" so the wire of the antenna has to be coiled up to fit the physical length that you require hence longer and higher is better both in terms of the coil (load) and position. Of course a longer division of the sign wave works better than shorter so a 5/8 wave antenna will put out slightly better than a 1/4 wave antenna, if you have a Firestik then you are probably using 5/8wave and that would be my weapon of choice.
catch you on the skip some time.
