None. The no ground plane antenna is internally grounded so the metal on the car does not interfere (well not much) with the signal plus I am using the 18' cable that came in the kit. I did tune the antenna using a SWR meter. Here is way more info than you would ever want to know about the system.
http://www.firestik.com/Tech_Docs.htm They come in red white blue and black
It really works superbly. I have no problems or complaints. I get much better range and clarity than when I taped a k40 on my car for BR2010
http://goo.gl/46H3h plus it kept blowing off all the time. There was some one on the run that mounted on a receiver hitch it is allot more sturdy for obvious reasons.
Hope that helps
