As I understand it, most mobile antennas don’t have ground plane radials because the
metal roof of the vehicle is the counterpoise. I mounted my mobile antenna in the attic above the garage. I started by putting it on a 2’ x 2’ steel plate but that wasn’t enough metal for a good counterpoise aka ground plane. SWR was too high, around 3:1. Ended up using sheet metal from Home Depot, because I just didn’t have anything else laying around. Need to have enough metal to simulate the roof of a car, 9 square feet or more, according to my research. The steel plate was approx $9, the sheet metal about $24. Later, I found a forum post and a separate YouTube video confirming that I could have used plain old aluminum foil. I suspect I could have attached magnetic base of the antenna to the steel plate, and set that in the center of an aluminum “blanket.” I’ve read posts about using a metal pizza pan or metal trash can lid, an old filing cabinet it dm old metal PC computer case. Folks claim these work but I don’t think it’s enough metal for the CB band. Might be ok for the 2 m and 70 cm HAM bands. Here’s a photo of mine, I’m getting an SWR of 1.5:1
View attachment 77463