What about the coaxial probe !!?

hi people, i ′ve been working latelly on microstrip p@ath antennas, and altough i′ve followed a coupled of books i still have some doubts about feeding...more precisely:
I′ve fed the p@atch with an inset feed, the proper width should have been 28 mm (by fomula & software: appcad;linegauge), but i ended up matching the antenna with an 18 mm microstrip and i did try several widths in between. The first one is a 50 ohms line, the second is about 64 ohms. (results from IE3D)
But if i feed it with a coaxial probe (50 ohms) at the exact same position as the inset microstrip it turns out matching the antenna, and damn 64 is NOT 50 !
Why is this happening ? all clues welcome

thanks in advance.

well, with probe feeding, you should move the probe into the center of teh p@tch (say, 10%~20%) to have a good matching. closed form equations are available for probe position. best book about is as anyone know : "Hanbook of Microstrip Antennas" by bahl and others.

marti