"David" nospam@nospam wrote in message
...
A normal ground plane is a large sheet of metal that reflects the radio
wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too much
of a gap for them to form a ground plane?

Effectively, yes.

A metallic surface (your "sheet of metal") can be replaced by a partially
metallic surface -- within limits. If you keep the size of any gap under
1/10 wavelength, the surface will appear solid. This I know from satellite
reflector work.

The use of four radials appears to be a compromise for using a solid
surface, but it obviously works. The RF sees these radial wires and behaves
like we want. I think adding more radials will always make a better
counterpoise, but I also think you reach the point of diminishing returns
pretty quickly. (We aren't the first ones to speculate about this, after
all :-)