Lets see if i can dust off and simplify some antenna theory. A ground plane is a flat horizontal conducting surface that serves as part of an antenna. The ground plane does not have to be connected to ground (no wires to the chassis are required). when an antenna is installed without a ground plane the effective power output, receiver sensitivity & transmission range of the antenna are dramatically reduced. Typically ground planes are sized with a radius of at least 1/4 wave length of the lowest frequency being transmitted or received. CB band range from 26.965 MHz for channel 1 to 27.405 MHz for channel 40. 1/4 wave for channel 1 is a little over 8.9764 feet. A null ground plane aka no ground plane antenna uses the shield in the antenna coax to provide a ground plane for the antenna. That is why the coax is precisely 17.9529 feet long. the reason we tend to mount a firestick on the windshield A pillar is mounting it on the roof means we are more likely to break an antenna because of the height above the roof of the coach.
When we are on a rolling rally coaches will frequently travel in groups of 3 or 4 coaches. For those coaches to communicate the choices are CB, Family Radio, Cell phone or smoke signals. Since we are frequently in areas with spotty cellular coverage that leaves us with either CB or Family radio's to communicate. When running a long rolling rally we tend to use all three over the course of the rally and in fact I carry 4 family radios so we have at least one way to communicate with the coaches that are traveling with us.
It is a complicated subject. Thanks for the information.
The CB instructions say, as you noted, I should use a 1/4 wave antenna but it doesn't specify a height as a requirement. As my coach's roof height is over 12-ft, I was only planning to get an antenna in the 3 foot height range (attached to the coil-spring mount).
But I still don't have the answer I'm looking for.
1) If the Coach was built for a "No-ground-Plane" antenna it would have the correct length coax cable already installed. So I just need to install a no-ground-plane" antenna.
2) If the coach was built for a "null-ground-plane" (has a metal plate under the fiberglass where the antenna is mounted), then I need that type antenna.
I can't find anything to support this now - but I think I read/saw somewhere that there is a large metal plate in the roof under the antenna mount location.
I'm going to call BCS today - if they can help, I'll post back what they tell me.