JB wrote:

The frame is the only substantial ground and certainly most effective for 40
meters. Use of the corners of the vehicle might actually get you a
counterpoise on 20. What you really need is a trailing wire, dragging a
cast iron stove.

At HF, a vehicle isn't "ground" or a "counterpoise", but the bottom half
of an asymmetric dipole. It radiates at least as much as the "antenna"
due to currents flowing downward along the outside. Calling a vehicle
"ground" or "counterpoise" doesn't impart magical properties -- it's a
conductor carrying currents whose fields don't cancel. In other words,
it's an integral, radiating portion of the antenna. You can't leave this
significant part of the antenna out of a model and expect the model to
give correct results.

And modeling a vehicle can be challenging because of the proximity of
conductors, particularly the whip and vehicle. You have to follow the
rules for closely spaced parallel conductors, and watch the average
gain. You might need considerably more segments than normal where
conductors are very close.

Roy Lewallen, W7EL