dansawyeror wrote:
Where can I learn about the difference in ground characteristics between
75m and 2m? . . .

If you want to model 75m ground, you need to begin with what you assume
the ground characteristics to be at 75m. Let's say you have average
ground, with conductivity of 5 mS/m and dielectric constant of 13. For
your model, you'll need something with conductivity of 5 * 146/3.8 ~ 192
mS/m conductivity and dielectric constant of 13. I have no idea where
you might find such a material.

William Orr wrote that a cross mesh under an HF vertical
approximated a perfect ground. It should be relatively easy to do the
same under a 2m model. I assume that metal reference ground would react
the same.

No problem approximating a perfect ground. If you have a perfect ground
under your 75 meter antenna, it won't be hard to make a decent model of
the perfect ground. A piece of screen or hardware cloth would do fine.
The trick will be making that perfect ground under your 75 meter
antenna. And unless you do, the model won't behave the same as the real
antenna.

Roy Lewallen, W7EL