Tuning mobile antennas...

What use is an F-call? I've recently purchased four verticals, one for each of the HF bands I'm allowed to use. I installed them and started playing, only to be confronted with some interesting results. The 80m and 40m verticals have a very high Q, that is to say, they resonate on a particular frequency and you can make contacts at those frequencies with about 22 kHz variation. This is as expected. My antenna analyser picture looks just like the one on the box. This is great. In theory all I should need to do is trim them a little bit and have them resonant at the frequency I want to operate on. I did hold off on the trimming, since cutting antennas longer is pretty hard to do, and because I got some weird results for the other two verticals. On 10m and 15m the antennas are resonant outside the band. On 15m it's below the band and on 10m it's above the band. I could just cut the 15m antenna shorter, but there is something strange going on here. I tried using different mounts, even a magnetic - not recommended - mount and still got strange things happening. I consulted some amateurs with more experience, one suggested that I remove the stinger and see if the result was in keeping with what was expected, that is, would it move the resonant frequency by the amount of shortening that removing the stinger would mean. Another explained that the roof of my car was getting in the way and that it was changing the characteristics of my antenna. I tried all manner of things, but trimming the stinger was not one of them. I'm glad I didn't. Yesterday I removed the 2m vertical from my house and as an experiment put the 15m antenna in its place. They're within 5cm length of each other, so the neighbours won't actually notice. I hooked up the analyser and found that it was perfectly resonant on 15m, worked with the same high level of accuracy that the 80m and 40m antennas represent. I also took the analyser for a spin up and down the band and found that my 15m antenna is also good for 2m and 70cm, so I don't have to pull it down every week while I experiment. I've yet to hear a strong enough signal on 15m to work anyone, but I'm not yet sure if that is because my antenna isn't working or because propagation is up the creek. I'm crossing my fingers it's the latter. I've not worked out what my car is doing to this antenna, but now at least I know that it's unlikely to be the antenna itself, which is great news. Off to do some more reading and consulting of fellow amateurs. Who knows, one of them might have some ideas. I'm Onno VK6FLAB