> How does a ground plane work?
> What are the benefits vs other antennae (discone ect.)?
> What freqs. do they work best on ?
I assume that you are probably referring to a 1/4 wave vertical antenna
supported over a conducting ground plane surface. This antenna has a
radiation pattern just like a 1/2 wave dipole. The ground plane creates
an image antenna, the other half of the 1/2 wave dipole. The impedance
of the ground plane antenna is 1/2 that of the regular dipole, which
would be about 35 ohms with some inductive reactance.
Its benefit is that you can get performance like a dipole, but you only
have to put up with half of the physical height. Also, if you have a
metal structure that would interfere with the vertical dipole, you can
use it to your advantage as a ground plane (like the roof of a car).
Sometimes, instead of a 1/4 wave element, a 5/8 wave element is used,
which theoretically concentrates the radiation pattern on the horizon
for maximum range.
These antennas are typically used at VHF and UHF, because at lower
frequencies, the required ground plane gets rather large.
Hope that helps, Dale