"Jim Weir" wrote in message
...
A "tuned" ground plane is one that is an odd multiple of a quarter wave at the
operating frequency. Odd -- one, three, five, and so on. In practicality
(with
the possible exception of GPS) the norm is ONE quarter wave, or a quarter-wave
groundplane.

Having said that, you must understand that you cannot make a "perfect"
circular
("disk") ground plane for a transponder,

Jim, I once built such an antenna (a 1/4 wave radiator with a 1/4 wave disk
ground plane) for a specific 800 MHz frequency and it was a disaster, with
almost infinite SWR at the cut frequency. I cut 4 large V's out of the disk
leaving 4- 1" 1/4 wave tabs, and I now had perhaps a 3 to 1 SWR. I finally
bent the tabs down 45 degrees into the usual "ground plane antenna"
configuration, and suddenly the thing had a near-perfect SWR with a wide
bandwidth.

I learned two things from that experience, 1) there is a lot I still don't
know about antennas, and 2) be cautious of tuned disk ground planes.

Perhaps you can tell me what went wrong.

Vaughn