Fred W4JLE wrote:
It would be interesting to recreate the measurements at other locations. My
location has 500 feet of sand below me. It would be a great improvement just
to have poor soil.

Depends on your objective. For NVIS operation with a horizontal antenna,
where you need the reflection, that's probably true. But for a vertical
or for DX with a horizontal antenna, you're better off with the sand.
Perfect ground has no loss; free space has no loss. There's an
intermediate quality of ground at which the loss is maximum at a given
frequency. Unfortunately, this happens to be in the range of ordinary
ground characteristics in the HF range. Your ground should be very low
loss. And your pattern should resemble free space, with a very strong
field at very low radiation angles.

Roy Lewallen, W7EL