If your antenna is not adjustable make sure you bring a way to cut it. Hack saw would probably be the easiest if it is a fiberglass whip or a big set of lineman pliers if is a steel whip
is yours an metal whip or fiberglass? Metal whip is easy..
Fiberglass--take off the rubber condom on the top of the antenna. Does it have a metal rod going into the center of the fiberglass rod that is secured by an allen head set screw? If not it is one that has to be cut to be adjusted..