There is no magic about 120 radials, and long before 120 radials are
reached the increase in field strength pretty much stops.
According to the 1937 BL&E experiments and IRE paper, this depends on their
length.
If they are short (~0.15WL), then there is little improvement in radiated
field when going from 30 to 120 buried radials. But that is not true when
they are long (~0.4WL).
This is nicely summarized in the clip below from a 1947 IRE paper by Carl E
Smith, a well-known broadcast consulting engineer.
http://i62.photobucket.com/albums/h85/rfry-100/GndSystemLosses_zps0b36c41e.jpg
RF
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