Thanks everyone.

In the end I solved the problem with two unused SMA connectors I
happened to find at the lab I work, to manufacture a cable to
reposition the antenna.
The only problem was the cable, cause I could not "borrow" a 2.4Ghz
cable as well.
So I simply tried with a coax BNC cable (the ones used for
oscilloscopes, if you used one). Soldering was a mess, especially cause
I had to invert the SMA central pin from male to female.

Being in an already good spot, I kept the cable very short. It's maybe
less than 2ft now. As soon as I connected it the signal was better, but
it will go down to nothing from time to time in an unrealiable way. So
my last guess before throwing everything away was to put some aluminum
foil around soldered spots at the cable ends, and that surprisingly
worked wery well. Now I consistently get a "very good" signal.
I may add a cantenna or parabola on either the AP or th receiver to
reach "excellent".

Thanks for all suggestions.

Stefano