The other night, after thinking about my results over the weekend, I read the following webpage and had one of those moments.
I went out and moved my cross bar, with the second NMO and 5/8 wave antenna attached, to near the center of the roof. On 5w I hit the Pikes Peak repeater and got a strong return signal from the repeater. So feeling encouraged, I then moved that whole set up to the very front of the roof so the bar and antenna were now centered between the sunroof and the windshield. I hit the Pikes Peak repeater again and had a nice talk on 5w with a very good signal report! So the issue was indeed lack of ground plane when my first test was setup over the sunroof. I really thought all that metal near the antenna would be sufficient, NOT. Common mode current is not your friend. The amount of sheet metal directly under the antenna is what is important.
Really good information in the link above although I think the discussion of the electrons moving is probably to make the concept more understandable since as written it seems to contradict the ARRL antenna resource "Why an Antenna Radiates".
Here is another good discussion on antennas.
I will be reconfiguring the mount to try and get it even closer the the roof.
Originally Posted by gahi
1/2 wave be any better? I'd be curious to see what its
specs for those are in the different mounting locations.
I'm just now working on a fender mount 1/2 wave single band. Switching from a rooftop mag-mount 1/4 dual band. I'm not dedicated enough yet to drill a hole in the roof.
Obviously I am not an expert. While the 1/2 wave does not require a ground plane weather it is better than what you are using will depend on several factors discussed in the links Dave and I posted. Specifically Dave's first link. All else being equal, a center mag mount is not a bad performer. Change antennas and it is a tougher question to answer. IMHO, the bottom line is the roof is the best place for an antenna. Obviously there are other considerations such as overhead obstructions but there are also many options for a roof mount YMMV.