"Chuck"
It would be more accurate to bill this as "Mediocre radial system vs 20 m
x 30 m solid aluminum ground screen ".

I suspect much the same results would obtain for the roof mount, even if
the roof were at ground level.
_____________

When the comparison is made between the roof-mounted version as shown in my
PDF and an earth-mounted vertical using a 2-ohm "broadcast quality" r-f
ground system of 120 buried radials each about 1/4-wave long, then the
advantage of the roof-mount version drops to about 5 dB.

N.B. these radiators are electrically very short. Using taller radiators
would reduce the performance difference.

RF