Francesco L. wrote:
"I was wondering if it`s possible to shorten the radials from the
standard 1/4 wavelength to another value, a smaller one."

Elevated radials are resonant, but the 20th edition of the ARRL Antenna
Book has Fig 47 on page 6-26 showing radials shortened by capacitive or
inductive loading.

Buried radials have their resonance swamped by earth conductance. Such
radials are surely effective when the greatest current drops nearly to
zero at the ends, farthest from the vertical radiator, No
interconnections far from the origins of the radials are desirable as
they only encourage circulating current which only adds to loss.

E.A. Laport has Figs. 217 and 218 on page 119 of "Radio Antenna
Engineering" taken from Brown, Lewis, and Epstein which show field
strength variies versus 2, 15, 30, 60, or 113 ground radials of 0.412 or
0.137 lambda, at antenna heights up to 100 degrees, so that you can see
what the shorter radials cost you.

Best regards, Richard Harrison, KB5WZI