Alejandro Lieber wrote in
:

....
The 8 radial can be as short as 4 feet long for the same results.
I came to these results from R. J. Edwards (G4FGQ) program
ENDFEED.EXE It's interesting to note, that radial longer than 1.3m (4
feet), always increase earth looses.

The model sensitivity to radial length suggests that the radials are
modelled as short transmission line immersed in a lossy medium, in which
case, the assumptions about soil conductivity and permittivity are very
important to the model accuracy. It is unlikely though that shallow
buried radials are very consistent in behaviour due to the effects of
rainfall.

It would be interesting to measure the antenna and compare it with your
model predictions.

Owen