Richard Fry wrote:

Below is a link to a calculator on the FCC website giving some
insight into this. It restricts inputs to values allowed for AM broadcast
stations, but still might be of some value to amateurs.

For one example, it shows a 1/4-wave monopole using 120 x
1/4-wave buried radials as generating a groundwave field of
about 306 mV/m at 1 km for 1 kW of applied power.

If the number of radials is reduced to 90, and their length is reduced
to 0.153 wavelength, the groundwave field is reduced to 267 mV/m.

The difference in radiated power then is (267/306)^2, or about
24%, which value is dissipated by heating the earth.

Whether or not that reduction is important to amateurs is a judgment
call. But an antenna system producing 267 mV/m for these conditions
would be unusable by a "regional" AM broadcast station -- which per
their station license must produce a groundwave rms field of at least
282 mV/m at 1 km for 1 kW of applied power.

http://www.fcc.gov/mb/audio/bickel/figure8.html

RF

This is a good example of why we shouldn't assume that what's suitable
or optimum for AM broadcasting or some other service is necessarily the
best solution for amateur applications. A reduction in radiated power of
24% is just about 1 dB. While this amount of attenuation makes the
system unsuitable for AM broadcasting, it would be difficult to even
detect that amount of difference except just perhaps in the most
demanding amateur communication -- right at the noise level -- and it
would go completely unnoticed in the vast majority of cases.

Roy Lewallen, W7EL