Owen Duffy wrote in
:

....
But...

What if ground resistance was 10 ohms, efficiency of the antenna and
loading coil would be 5/(10+5+0.7) or 32% (5dB loss). That will help
the line loss though, now with a feed Z of 15.7-j22, loss in the same
line is 1dB and input Z is 16+j2. ATU loss might be more like 0.4dB.
Total loss is 5+1+0.4 or 6.4dB.

I didn't want to give the impression that this isn't a significant
improvement over the unloaded vertical on 7MHz.

The Force12 page give an analysis and loosely claims 16dB improvement
over the unloaded antenna, but that is probably based on ignoring ground
resistance again.

Assuming ground resistance of 10 ohms, feedpoint Z would be 15-j460,
ground loss 5dB, line loss 12.2dB, ATU loss ~1.1dB for a total system
loss of 18.3dB.

Improvement from system loss of 18.3dB to 6.4dB is 12dB, a little less
than they might want you to think, but very worthwhile.

Of course results are very sensitive to the ground scenario.

Owen