Danny Richardson wrote:

Yep. Put a 1/2-wave dipole 1/2-wavelength over sea water and compare
its gain to the gain to a isotropic in free space.

Actually, the gain of a horizontal antenna at reasonable height and
reasonably low angles is nearly the same over plain ground as it is sea
water.

But do what Danny says, and you'll find that a dipole over ground has a
gain of around 6 - 8 dBi. While dBi is rigorously defined, "dBd" isn't.
Using a common conversion of 0 dBd = 2.15 dBi (the gain of a dipole in
free space), you find that a dipole over ground has a gain of about 4 -
6 dBd. At least one major antenna manufacturer quoted the gains of their
antennas over ground in "dBd". Naive people assume that it means
"compared to a dipole at the same height", while it actually means
"compared to a dipole in free space". Gives their antennas an instant 4
- 6 or so dB boost over the competition.

Roy Lewallen, W7EL