I can beat their prices!
The size of the circles
determines the lowest frequency the antenna works at.
How narrow you can construct
the slot between the two disks, or disk ground plane,
determines the highest frequency the antenna works at.
At the low it behaves like a
fat dipole or 1/4 wave ground plane.
At the upper end it turns
into two exponential slot, or Vivaldi antennas looking off to
So at the low end your have
an omni pattern.transitioning to an oval pattern.
Properly done they never
null. I rate them to 26 GHz and that is really the frequency
limit of the SMA connector.
With 'K' connectors they are
commonly used at 35 GHz.
You can go down to Tesco or
Poundland. Look for some pizza pans about 40 to 50 cm
across. Mount them a millimter or two apart. Solder your
coax across that gap. You now have a 2 Meter though 23 cm
antenna. 13 cm if you are careful. 73 Kent
PS The basic design goes back
to the 1930's