I have a Nissan Versa 2012 (hatchback). It comes with one of those default antennas at the back/center of the roof, for listening to AM/FM radio stations.
I have a dual band mag-mount (TRAM 1185), which I placed right in the center of the roof, with the coax entering through the trunk.
It is working OK. But I am new to the hobby, so I am wondering if the exact center of the roof is the best place for an antenna, given the placement of the default AM/FM antenna.
When dealing with UHF and VHF communications, height is often the determining factor of range for line of sight conditions. If there are obstructions that height cannot overcome, then more effective radiated power tends to be the determining factor.
Exact placement on the roof won't have much effect on the pattern. If you place it way to one side of the roof, then you may have a signal that is a bit lopsided due to the lack of a ground plane in that direction.
The question often comes up about any potential damage of your AM/FM radio by your ham radio, consider that your FM broadcast radio will have more than 0.5 volts of an in-band signal when you are within 1/2 mile of a 50 kW FM station and this doesn't cause any harm to the radio. The front end filters of the FM receiver will likely attenuate out of band signals by more than 40 dB. For your 2 meter transmitter to pass the equivalent voltage through the front end of the FM broadcast receiver when the antennas are located 2 feet apart would require more than 200 watts of power. So your 2 meter or 70 centimeter transmitter may cause some minor temporary overload of the broadcast receiver but it is unlikely to cause any harm. I regularly run 50 watts and have never had a problem.
It's difficult to predict in the general case how a particular antenna placement will work because it's very sensitive to the particular geometry of the car. Most likely the center of the roof is just fine, and alternative placements would yield not very significant differences.
