Rik Strobbe wrote:

Looking at the patterns:

am I right to conclude that not-so-perfect ground will only attenuate the surface wave (radiated at 0°) but will not affect skywave (or even improve it a bit)?

I don't know... But, experience from experiments, software simulations and
reading a lot of books converge into the fact that vertical antennas over
non-perfect ground do not radiate well at zero elevation. I believe it
has something to do with the GND beeing a not-so-perfect mirror which,
in addition to beeing lossy, also has some phase shift in its reflection
coefficient. I guess...

I think that most of this attenuation happens in the near field region
which I believe is exactly what Jim M0BMU has found in his experiments;
the FS falls off almost linearly with distance but it is a few dBs
"too low". An FSM antenna at 2m height is below 1 degree elevation
already at 114m distance from the TX (assuming that the Earth is flat,
isn't it? ;-) )

73
Johan SM6LKM