"Interesting post... can we assume the 16 radials are 1/4 wave as well ?"
no we cannot as the velocity factor of a conductor either laying on the ground or buried beneath the surface is substantially shorter than a physical 1/4 wl.. the velocity factor is modified by the increased inductance between the soil and the radial conductor/s (insulated or not) coupled with the unique conductivity and dielectric permittivity (R=100, K=12) of the soil present at the antenna site.
"I would estimate that you would need something like 60 radials, give or take, to get the results that the op is posting."
if you're assuming that the increased directivity is produced in large part due to the effect of the number of radials then your assumption is wrong. the radiation efficiency of the antenna when moving from 16 to 60 radials produces only a 0.36% increase in radiation efficiency while reducing rf loss resistance in the ground electrode system by a mere 0.3 ohm. there's not enough difference there to justify all of the extra wire and labor.
no where did i state that there were 16 radials used in the ground mounted verticals represented in the graphic appearing in the op.
read it again.
a ground mounted 1/4 wl. vertical will outperform the same antenna with 3 or 4 radials elevated to any feedpoint height up to 54' and demonstrate a clear advantage in directivity of better than 3 dB. using a minimum of 16 radials, either laying on the ground or buried. it is also immune to the re-radiation of antenna currents flowing on the outside of the shield, eliminating any necessity for rf chokes, current baluns and doing away with the requirement for isolation from any conductive, elevated support structures. antenna radiation efficiency is also about 20-25% greater than its elevated counterpart, which directly results in higher levels of power radiated by the antenna and increased directivity and yes, now we can talk about gain.
radiation efficiency X directivity = gain
radiation efficiency is 95.96%
.9596 X 7.5 dB. directivity = 7.197 (7.2)
dB. gain @ an ert angle of 45 degrees.
the difference in directivity between the two grids with respect to the comparison between the ground mounted 1/4 wl. and the ground mounted 3/4 wl. verticals is less than .7 dB.. the distance between the "circles" in the both of the elevation display grids represent a change in directivity of 3 dB. along any radius or specific angle of radiation.
