Think of your ant. as having two parts. The top or hot side is above the bracket. The neg. or ground plain aka truck, car, is the other half. The ant. should be attached to the ground plain, and the coax should be attached to this point.

Your truck acts like a counter poise or a reflecter for the signal, so the closer the ant. to the ground plain, and the posistion of the ant. is very emportant.

Because todays semi trucks have a lot of fiber glass and you have to put your ant. way out on the mirror bracket, away from the optamum center of the truck, it becomes less efficiant.