FIG. 1 shows the relative gain (loss) of an antenna (e.g., dipole or beam), under varying ground conditions, through the frequency range of 5-30MHz:
A represents the curve under perfect ground conditions, B under average ground conditions (i.e., wet grassland), and C under poor ground conditions (dry desert).
Note that the influence of ground conditions diminishes rapidly as the frequency increases.
While a good ground system can substantially improve performance at the lower frequencies, at 30MHz there is very little difference in gain between a dipole with a perfect ground and one with poor ground.
For example, at 5MHz a receiving or transmitting signal that is S9 +10 dB over perfect ground becomes S9 +7 dB over extremely poor ground (a 3 dB drop); but at 30MHz, a signal that is S9 +10 dB over perfect ground, drops to S9 +9.4 dB over extremely poor ground (a drop of just .6 dB!).
Conclusion: The influence of ground conductivity becomes less important at 14MHz and higher (1.2 dB difference or less).
A typical 3-element Yagi 1/2-wavelength high over perfect ground has approximately 14 dBi gain (as compared to an isotropic radiator in space).
Information from “Neues von Rohde und Schwarz” Oktober/Nov. 1973.