The link below leads to a graphic on the FCC website showing the groundwave
field strength produced by 1kW from vertical radiators working against a
radial ground system of 120 quarter-wave radials; the typical AM broadcast
setup. The field strengths in the table do not include the affect of ground
conductivity. They are related only to the "efficiency" (an FCC definition)
of the radiator against the stated ground plane.

Field strengths in the FCC chart range are empirical values, and range from
~190 mV/m for a 0.05 wavelength vertical to ~641 mV/m for a 0.625 wavelength
vertical. This is a considerable range of field strength values for
matched, simple antennas all driven with the same tx power.

Statements on this newsgroup about the free space gain of a very short
doublet antenna being not much less than a full-size 1/2-wave dipole might
lead some people to conclusions different than shown in the FCC chart.

"FWIW."

http://www.fcc.gov/mb/audio/decdoc/s...3190fig8/2.gif

RF