J&M supplies antennas that are already SWR'd and work fantastically well. I have a 2' and a 3'.
From what I have learned, No CB antenna is, (already SWR'd). They all have to be adjusted on the bike using an SWR meter. That goes for both the Ground Plane and No Ground Plane antennas. The only antennas that don't need adjusting are the (rubber duck) antennas on a portable hand held CB.
This from a major CB radio company.
"The Antenna must be tuned for all applications. Factory systems, aftermarket ad on’s, replacement antennas, it makes no difference. The antenna SWR MUST be set! There is no pre-tuned antenna."
I just found this on the J&M web site.
Note: This antenna has been designed to mount to your motorcycle easily and
attractively. The antenna staff has been pre-tuned specifically for use with J&M’s
JMCB-2003 CB. No further tuning is required.
Every thing I have learned says that a CB antenna must be adjusted for each installation.