Why is there a grid below the antennas of an VOR?
I think this is an example of a counterpoise:


  [It] is a network of suspended horizontal wires or cables (or a metal screen), used as a substitute for an earth (ground) connection in a radio antenna system (...) when a normal earth ground cannot be used because of high soil resistance or when an antenna is mounted above ground level.


Generally used for LF and VLF antennas as that is where they are most efficient.

An example from that Wikipedia page of an AM radio antenna counterpoise: