Thinking about this and Dick's description of his setup (hamstick on a tail light mount), I have a few questions and a speculation:
- What does the tail light mount look like? I presume it's a bracket that sticks out between the tail light lens and the metal body of the car.
- Which direction does it stick out--to the back or to the side?
- Which tail light is it on?
- How much of the hamstick extends above the top of the T&C minivan? (Or what fraction of it is below the roof line?)
I wonder if part of this could be the direction the car is pointing and thus the location/orientation of the ground plane, which is affected by the answers to the questions above. It might be an interesting experiment some time to find a large empty parking lot and ask Kathy to drive around the perimeter so that the car ends up rotating 360 degrees (multiple times). If we had multiple stations listening, all in different directions, we might be able to get an idea of the pattern the antenna-on-car generates.
And if someone is good with antenna modeling software, we could compare the experimental pattern with an analytical prediction.