Patrick,
Sorry, tuning the antenna 'off of' the car won't work
(miracles
do happen, but don't count on it). The most likely problem is
as
you thought, a not so good 'ground', but maybe not. Does
anything
you do change the tuning of the antenna at all? Maybe not get
it
to where you want it (freq.), but does change it's resonant
freq?
If not, then there's something else 'going on' besides the
inadequate
grounding/counterpoise. Make sure you have a good connection
through
the mount, no shorts/opens except as they should be, etc. And
you
probably should do something about the grounding/counterpoise,
maybe
clip a wire to the base of the mount as a counterpoise to see
what
difference it makes (just lay it on the surface of the car for
now).
The length of the counterpoise isn't that critical, but too long
is
'better' than too short. Changing the location of the mount is
also
something you might try.
Good luck...
'Doc