Sirius antenna mounting question?

gozfast

I installed a Sirius receiver with my Sony HU and put the antenna inside the car on the right front dash with just some double sided tape. The reception is absolute crap. Even the smallest trees next to the road will interrupt the signal. I see some of you have a metal plate under the antenna on the dash. Is this required and where would I get one or should I just make something? Would putting the antenna on the outside of the car help? The roof's not made of steel either. I have Huper Optik film on my windshield but I thought that was designed not to interfere with signals. A little help getting this thing to work would be great because I really HATE the commercials and the local radio stations here. My new stereo system sounds great but I can't stand to listen to it.