Ok,

w = c / f; so w= 1 wavelength - use c = speed of light (in air)

s = 0.487 w ; where is the 0.487 from?

I'm also confused why "split the difference" - ie. 1060Mhz, at 1090
Mhz the wave is shorter, and when the sides are cut as you describe,
one ends up with the shortest length = s, and the "tips" of the
octagon across the piece are longer....I would expect that one would
desire a piece that has a section that is as short as the 1090Mhz, and
sections that are as long as 1030Mhz...

I think I am still missing something here....

-------
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, as it must operate on two frequencies
simultaneously -- 1030 and 1090 MHz. What is quarter wave for one is not
quarter wave for the other.