Hi Boy and all,
Since the mounting is unpredictable so will be the results, no practical answer forthcoming. Just because you have a good match with the CB antenna one time doesn't say it'll be good the next time, antennas are pretty finicky when disturbed. BTW, a mag mount HF (CB) antenna is what we call a dummy load regardless of SWR, there is radiation resistance and overall efficiency to consider and capacitive coupling to ground doesn't cut the mustard on HF. Although it has nothing to do with coupling placement is critical even at VHF. A friend used a 2M mag mount and the SWR changed every time he stuck it to the roof, ground plane geometry also comes into play.
Well, since permanent mounting is not an option you'll just have to expect unpredictable results but then again predictibility isn't really all that important in a mobile where everything changes when the vehicle is in motion. After all it's a hobby, not a science.
