I know of commercial AM broadcast stations using 6-8, ~1/4-wave radials
elevated less than 20 feet over (rocky) ground, with an antenna system
radiation efficiency meeting the FCC minimum for broadcast station use.

wrote
How does that minimum compare to the usual 120 or so radials
on the ground?
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Here are the minimum h-plane, r.m.s. fields required by the FCC for
commercial AM broadcast stations of the various classes at 1 km, for 1 kW of
applied power.

1) Class A: 362 mV/m
2) Class B: 282 mV/m
3) Class C: 241 mV/m

That value for a typical 1/4-wave vertical monopole with 120 buried radials
each at least 1/4-wavelength is about 306 mV/m (regardless of frequency and
the ground characteristics at the antenna site).

The theoretical maximum h-plane r.m.s. field at 1 km from a perfect 1/4-wave
monopole over a perfect, flat ground plane is about 313 mV/m for 1 kW of
applied power.

RF