Cab-over-campers used on pick-ups with mirror mounted CB antennas can create performance problems. In many instances the antennas were originally mounted when a smaller camper was being used or when the cab-over-camper was sitting on its stilts in the back yard. After installation the antennas were tuned and everything was fine. However, after the cab-over-camper is mounted you may notice a loss of performance. And, if you rechecked SWR you might be surprised to see that it is in the unacceptable range.
SINGLE ANTENNA INSTALLATIONS
Regardless of left or right side installation, when the antenna is on the mirror and the camper is removed, the antenna is far enough away from the vehicle body and high enough to keep the highly active upper portion of the antenna in free space. But, as soon as you install the camper, the upper potion of the antenna is too low and too close to the camper. In best cases, SWR will be between 2.0:1 and 2.5:1 and you will notice poor performance and dead spots towards the non-antenna side of the vehicle. In severe cases the SWR could reach a critical ratio (indicates that high amounts of radiated energy is reflecting back into the transmitter) and the radio could be damaged. In any case, the antenna needs to be relocated. There are three possible solutions.
DUAL (CO-PHASED) ANTENNA INSTALLATIONS
Co-phased antennas rely on each other to reflect the transmitted energy. In order for co-phased antennas to work with any amount of efficiency, the antennas MUST have clear, unobstructed line-of-sight between one another. That just can't happen with antennas on the mirrors and a cab-over-camper in place. If you insist on having dual antennas you will need to mount them on the camper (and loose the use of them when the camper is off), have a set of dual antenna on the camper and on the truck, use dual hood channel mounts on the forward half of the vehicle hood, go to a single hood channel mount on the forward half of the vehicle hood, or create a double usage system of dual and single antenna installations.
In any case, do not ignore the potential problems created by a large cab-over-camper. Whenever you change the surroundings in the antennas near field of radiation you must check to see if those changes affect the system SWR. And don't forget that if the antenna was tuned with the camper in place, you should also recheck it after the camper has been removed. In these on-again, off-again situations we highly recommend using our tunable tip antennas. With the antenna tuned under one set of conditions (i.e. with the camper off) you can measure the tuning screw height and save the dimension. When you return the antenna to that particular location after the originally tuned condition is repeated you can easily set the tuning tip to its proper height. That will save you from having to re-measuring SWR each time you make a change.