Clarification of solid ground plane (antennas)
It all comes down to the wavelength (frequencies) involved.

If the openings between the rods in the antenna you showed are small compared to the wavelength, they effectively look like a solid plane.  Openings less than one quarter of a wavelength start to block RF energy.  Openings of less than 0.1 lambda (wavelength) behave like a solid piece of metal, with a little bit of RF leakage allowed through.  The smaller the opening, the less RF gets through (or to say it another way, the more the signal is attenuated).

Look at this picture of an ARSR-4 (Air Surveillance Radar).  The operating frequency for this is 1.2 GHz to 1.3 GHz, with a wavelength of ~9.75 inches.  Notice that the reflector is not solid, but rather is a mesh.  The mesh reflector behaves just like a solid one would for all intents and purposes.