antenna "ground plane" question

I have seen posts saying an antenna needs a "ground plane" to perform optimally. I inquired earlier about buying a "mirror mount" (aluminum mirror on a semi) setup. I went ahead and bought a ProComm kit (model JBC290MM). I am listening to it now on my Pro-197 I am using as a base in my apartment. I am currently receiving everything clear, even agencies whose dispatches are located about 35-40 miles from here. My question: does anyone have experience that says once I mount it to an aluminum mirror (on my semi), that it may not work as well as it is now? If it doesn't, I am thinking about getting a BNC "T" splitter and using BOTH my Radio Shack magent mount (I am using now and it is magnetic mounted on the truck frame behind the cab) and this new JBC. Obviously I wanted a mirror mount as the antenna is up higher and out "in the open" (not low and hidden behind my cab). Thoughts are appreciated.......



Thank You all.......

antenna "ground plane" question