Originally Posted by bmsc
There is no need for a ground plane at those frequencies. At 600 Mhz the ground plane is less than 5 inches in diameter. At 2.4 Ghz it is about an inch. Almost all antennas today are broadbanded enough to not need a ground plane. I sell a lot of radio at 150 Mhz and they are now manufacturing antennas that don't need a ground plane.
I think you're right, the antenna being used here probably doesn't need it but it's not so much the fact that it "doesn't need a ground plane" it's more that clever antenna design has squeezed enough of a ground plane or something into the internal design. I think that's the case here with the disk shape.
I just re-read the manufacturer's documentation
on my Poynting and it says it is "ground plane independent" because it has an internal ground plane which seems different to some other documentation on it I read a while ago which suggested a few db better gain with an external ground plane. I'm going to use a metal plate as part of the mounting so I might as well continue with it.
The traditional general rule of thumb seems to be that the ground plane should be a minimum of a 1/4 wavelength radius. A wavelength at 600 MHz is half a meter so that means a ground plane of about 5 inches radius, or 10 inches diameter. I guess clever design and some compromise can probably get that smaller but it's not a matter of it being "broadband enough". It's still an electrical circuit that needs (and will find) a return path. If there's no ground plane or other side of a dipole or something, then the braid of the coax effectively goes nowhere which is not good.
One of the "problems" with antennas is that they don't have a binary "it works" or "it doesn't work" scenario. Almost anything will "work", it's just a question of how well.