NEC4.2 gives some insight into this ...
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A common belief is that Earth conductivity mostly affects the "takeoff angle" of a vertical monopole.
However the NEC4.2 study below shows that the shape of the radiation patterns is quite similar when the same monopole system is installed at sites where Earth conductivity differs through the range from poor to excellent.
The biggest effect when going from poor to excellent Earth conductivity (other things equal) is due to the reduced loss of the antenna system path to r-f ground (Earth), when the radial wires are buried in Earth having greater conductivities.
It is mostly this reduced loss that is responsible for those greater low-angle fields — not to a lower takeoff angle.
In this study there is approximately 1.8dB more power radiated in the elevation angle of maximum directivity for 30 mS/m Earth than for 0.5 mS/m Earth.
